Washington (CNN) Former Veterans Affairs Secretary David Shulkin has repeatedly insisted that the White House fired him from his job at the department. The White House says Shulkin resigned.

Which of those two things actually happened could have sweeping implications that could throw an agency beset with challenges even further into turmoil.

When President Donald Trump announced on Twitter last week that Shulkin would depart the agency and that a Defense Department official, Robert Wilkie, would serve as acting secretary, he side-stepped Shulkin's deputy, Thomas Bowman, who was next in the line of succession. That decision has given new life to the legal debate over how a President can pick a replacement for departing Cabinet secretaries.

What is the Federal Vacancies Reform Act?

The Federal Vacancies Reform Act of 1998 gives the President the authority to temporarily fill a vacancy at a federal agency if the official "dies, resigns, or is otherwise unable to perform the functions and duties of the office."

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