the Antideficiency Act riffing off the Constitution requires that, when there is no authorized appropriation, no money can be spent by a department except for functions that protect health and safety of the public. Certain people are designated to come in in order to protect H&S. A bare minimum of people who don't affect H&S can be brought in to support those who do directly (like bringing one person from the HR office). The TSA line inspectors and the FAA controllers were brought in on the H&S basis, but fewer of them than normal. Same thing was true for USDA Food Inspectors, FBI, ATF, and DEA Agents and the Deputy US Marshals. Some functions continued because "user fees" pay for the function, like the SEC inspectors at the stock exchangfes, otherwise they would have been required to shut down, too.