Essential versus nonessential. For millions of federal workers, it’s a critical distinction, and one that agency inspectors general are likely to take a close look at once the government shutdown ends.

“There’s a lack of clarity about who constitutes an essential employee and who is not,” said H. David Kotz, inspector general of the U.S. Securities and Exchange Commission from 2007 to 2012 and now a director at the Berkeley Research Group.

Under the Antideficiency Act, only government workers required to deal with emergencies “involving the safety of human life or the protection of property,” are supposed to be on the job, albeit without pay for now. All others are furloughed, a prospect that became more appealing to workers after the House on Saturday voted, 407-0, to give back-pay to those employees.

The penalty for violating the act is substantial. According to the Government Accountability Office, “employees may be subject to appropriate administrative discipline including, when circumstances warrant, suspension from duty without pay or removal from office. In addition, employees may also be subject to fines, imprisonment, or both.”

During the near-shutdown of 2011, Kotz said, SEC managers appeared to “designate as many people as possible in their offices as essential, in an effort to show they were an important division,” he said. “On the other hand, [workers] may want to be nonessential to have what amounts to a free vacation.”

The SEC is still operating, using carryover funds from its fiscal year 2013 budget.

Agencies “need to have good, strong watchdogs and oversight” to make sure they apply the Antideficiency Act correctly, Kotz said.

Some agencies have taken a very strict view of who is essential. At the Federal Communications Commission, for example, only 38 of 1,754 employees are still on the job, according to the agency’s shutdown plan. Those exempted from furloughs include workers who operate a high-frequency antenna system and conduct interference detection and disaster-response operations.

At least two courts, by contrast, have declared that all their employees essential: the U.S. District Court for the Southern District of New York and the U.S. Court of Appeals for the Second Circuit.

Courts don’t have inspectors general and taxpayers likely would lack standing to challenge the decision, said Boris Bershteyn, former general counsel of the Office of Management and Budget, now of counsel at Skadden, Arps, Slate, Meagher & Flom.

“It’s hard to get into the courts to challenge the funding of the courts,” he said. “If an inquiry into the courts’ decision were conducted, a likely forum would be a congressional hearing or investigation. But even that could raise important separation-of-powers questions.”

As for individual employees, if one wanted to challenge a furlough by arguing that his or her work was (or was not) actually essential, it would be a difficult case to make, Bershteyn said. “It would be an uphill battle, because courts will give a great deal of deference to the judgment of the agencies,” he said “It may not even be a reviewable decision.”

John Mahoney, who leads the labor and employment law practice group at Washington’s Tully Rinckey, said it was unlikely any major litigation would arise from the shutdown when it comes to essential versus nonessential determinations.

Ultimately, the decision is made by the agency head and general counsel’s office in coordination with senior management, but there’s not a lot of wiggle room, Mahoney said.

“Either [they’re] performing one of those essential functions or they’re not,” he said.

The Senate must still authorize back-pay for furloughed workers, but even if that doesn’t happen, the lost pay likely would not be large enough to make it worth the cost of a lawyer to fight that essential/nonessential decision, he said.

Also, if people deemed essential aren’t getting paid on time, they likely would lack standing for a lawsuit because, ultimately, that’s a legislative decision, Mahoney said. “There’s really no dispute the essentials have to be retroactively paid for this time they’re working for free,” Mahoney said.

Another issue Mahoney highlighted was the use of government credit cards for per diem expenses and hotels for travel by essential employees. Federal employees are required to pay those credit card bills on time, so the bills could come in the mail but they wouldn’t have the money to pay.

“If it drags out that long, how are they going to pay their bill?” Mahoney said. “And will they be disciplined if they don’t?”

Contact Jenna Greene at Todd Ruger contributed to this report.