When a shutdown occurs, DHS sends most “non-exempt” workers home within four hours. “Exempt workers” remain at work, even though they may not be guaranteed to get paid.
There are two main groups of exempt workers: Those involved in the protection of human safety or of property, and those whose positions are funded by fee revenues. Because of this, some immigration components will be spared the worst of the cuts. Among the Border Patrol’s employees, for example, 7,000 out of 59,000 were expected to be furloughed. Those not being released from work would be primarily agents guarding the borders.
Immigration and Customs Enforcement (ICE), the interior immigration police, would have been hit harder with an expected 4,000 out of 20,000 sent home. Many of those released would have been ICE attorneys, the lawyers who serve as prosecutors in the immigration courts. ICE arrests would have continued, but the courts would have virtually shut down.
The United States Citizenship and Immigration Service (USCIS) is in charge of judging citizenship cases, work authorizations, and many petitions for permanent residence. It is mainly funded by fees from immigrants, their families and employers. That means that most positions there are exempt from the layoff. At USCIS, only 350 employees would have been sent home out of more than 12,500.
The DHS crafted a detailed shutdown plan, which you can read here.