Former Vice President Kamala Harris has Secret Service protection, at least for a few more months.

Chelsea Clinton does not have a Secret Service detail anymore, though her father, former President Bill Clinton and his wife, Hillary Clinton, do.

All of President Trump’s family members currently have protection, including his grandchildren.

And while former President Joseph R. Biden Jr. and his wife are by law allowed to have protection for the rest of their lives, their adult children had it only for a few months. After leaving office, Mr. Biden issued an executive order that extended the protection to them, but Mr. Trump revoked their detail in a pique on Monday.

The controversies and Mr. Trump’s announcement have thrust the subject of security for public figures into the headlines. But they also raise questions: Who gets Secret Service protection? For how long? Who makes those decisions? How much is it costing the taxpayers?

Here are the ins and outs of government protection.

Who is the Secret Service required to protect?

The Secret Service is statutorily required to protect certain people, including the sitting president, vice president and their families.

In the case of both Mr. Biden and Mr. Trump, their family members made up a majority of people with Secret Service protection details during their respective administrations. Both men are grandfathers and have large families that have been under protection. Mr. Trump has five children and 10 grandchildren. Mr. Biden’s two children and seven grandchildren also had details while he was president.

The agency is required to protect former presidents and their spouses for life, and children of past presidents retain their Secret Service details until they turn 16.

In addition, visiting heads of states and their spouses receive protection details. So do major candidates for president and vice president and their spouses beginning 120 days before the general election.

How is protection assigned?

A president can also assign Secret Service protection through executive orders, meaning the president can add or remove the detail by fiat.

While president, Mr. Biden extended the protective detail for Mr. Trump’s youngest son, Barron, after he turned 16, according to an official familiar with the arrangement who was not authorized to speak about the matter and spoke on the condition of anonymity.

And before he left office, Mr. Biden also extended the protective detail for his vice president, Kamala Harris, for at least six months, the official said. Because the law does not require protection for former vice presidents, Mr. Trump could choose to end the security detail for Ms. Harris.

Toward the end of his term, Mr. Biden issued an executive order extending security details for his children, Hunter and Ashley, for six months. Mr. Trump did the same for his five children at the end of his first term.

But Mr. Trump abruptly ended the protection for the Biden siblings on Monday, speeding up a process that was already set to happen.

On social media on Monday, Mr. Trump wrote that Hunter Biden had an 18-person protective detail providing security while he was on vacation in South Africa.

“Please be advised that, effective immediately, Hunter Biden will no longer receive Secret Service protection,” Mr. Trump wrote. “Likewise, Ashley Biden who has 13 agents will be taken off the list.”

For security reasons, the Secret Service would not say how agents are transitioning out of protecting former President Biden’s children. The agency does routine security assessments of the people it protects, but it would not discuss the threats facing Hunter and Ashley Biden.

Anthony Guglielmi, a spokesman for the Secret Service, said on Monday evening: “We are aware of the president’s decision to terminate protection for Hunter and Ashley Biden. The Secret Service will comply and is actively working with the protective details and the White House to ensure compliance as soon as possible.”

A representative for the Biden family declined to comment Monday.

How much does it cost taxpayers?

The number of people under Secret Service protection fluctuates, and the complete list of people is not made public for security reasons.

The list grew to 54 from around 26 after the Sept. 11 attacks, according to a former official familiar with the protective details at that time. The official spoke on condition of anonymity to discuss information that is not routinely publicized.

During the first Trump administration, 42 people had Secret Service protection. That number dropped to around 36 while Mr. Biden was in office. Currently, the Secret Service has protection details for 43 people.

Hunter Biden’s travels raised questions about the taxpayer cost and necessity of a large entourage when the Secret Service is struggling with staffing and retention, former agency officials said.

“During a time where resource constraints are difficult in the Service, this is something that he just deemed as no longer the extension of a courtesy,” Ronald Layton, a 26-year veteran of the Secret Service who led divisions with oversight of protection, said of the president’s decision. Mr. Layton said cutting the protection details short was “common sense.”

While the agency does not break down the cost of protective details for each person it is assigned, the Secret Service’s total budget in 2024 for its protection-specific mission was nearly $1.5 billion, according to data from the Congressional Research Service. In 2019, which was not a presidential election year, the budget for the protection-specific mission was nearly $970 million.

Resources at the agency have been stretched thin after years of staffing shortages and the grueling 2024 presidential campaign season, which called for more protection for candidates than in any previous campaign cycle, partly because of the number of candidates who received protection and threats against Mr. Trump.

This came into stark focus after the attempted assassination of Mr. Trump last summer. The Secret Service was broadly criticized for its security failures at an outdoor campaign rally in Butler, Pa., where a would-be assassin was able to climb onto a building and shoot Mr. Trump.

How has Trump used his Secret Service power?

Despite the legitimate questions of cost for protecting so many people, the sudden announcement of the cessation of protection for Hunter Biden, coupled with Mr. Trump’s fixation on the former president and his only living son, raised immediate questions about whether this move was the latest stop on the president’s revenge tour.

It prolonged Mr. Trump’s controversial pattern of using the power to assign or dismiss security details in ways that highlight his personal grievances and potentially expose his perceived enemies to peril — part of a retribution pledge that he has effectively carried out since his return to the Oval Office.

Within hours of taking office, Mr. Trump removed the security detail of John R. Bolton, one of his former national security advisers. Mr. Trump fired Mr. Bolton in 2019, and Mr. Bolton later wrote a book critical of Mr. Trump.

Mr. Trump also pulled the protection for Mike Pompeo, one of his former secretaries of state, and a former aide, Brian Hook. Mr. Trump did this even though Mr. Bolton, Mr. Pompeo and Mr. Hook remain under threat because of actions they took during Mr. Trump’s first term.

As president, Mr. Biden gave Secret Service protection to Mr. Bolton, and later Mr. Pompeo and Mr. Hook, because of Iranian threats. Mr. Bolton and other officials from Mr. Trump’s first term, including the president, have faced continued threats for their involvement in an airstrike that killed Qassim Suleimani, a top Iranian general, in January 2020. In 2022, the Justice Department charged a man with plotting to assassinate Mr. Bolton.

When Mr. Trump pulled the protection details for Mr. Bolton and the other two former advisers, two Republican senators — Tom Cotton of Arkansas, who is the chairman of the Intelligence Committee, and Lindsey Graham of South Carolina — urged him to reconsider.

“Politics should not come into this in any way,” Jason Russell, a former Secret Service agent, said in an interview with CNN on Tuesday. “It’s an unfortunate reality.”

“In this case, it seems like a retribution or some kind of political gamesmanship to take protection away,” Mr. Russell said. “But the Secret Service really shouldn’t be used in this manner. It should be always a threat-based decision.”

Mr. Trump also stripped the government-funded security detail for Dr. Anthony S. Fauci days after his return to office. Dr. Fauci did not have a Secret Service detail, but he continued to have taxpayer-paid security after he left the government in 2022. Initially, federal marshals protected him, and later he had a private contractor whose fees were paid by the government. Conservatives widely criticized Dr. Fauci because of his role guiding the country through the coronavirus pandemic, and he had a contentious relationship with Mr. Trump.

Mr. Trump’s announcement on Monday did more than eliminate Hunter Biden’s detail; it also told the public where he was, potentially putting him at risk.

The vacation was not exactly a state secret, however, as The New York Post had published an opinion essay on Saturday about his travels.

“We are in a very unstable security environment where there has been a lot of political violence,” said Representative Jamie Raskin of Maryland, the top Democrat on the House Judiciary Committee. “Decisions about the extension of Secret Service protection should be made with discretion so people are not exposed to any added risks.”

Minho Kim contributed reporting.