From this:

Prince William supervisors Tuesday are expected to put in place a new directive from Homeland Security that will standardize how 287(g) immigration policies are implemented around the nation... ...The new agreement states that "ICE must give approval for an immigration arrest where no state or local charges are made [and] ICE must give prior approval of planned operations where immigration enforcement is the sole function." One more revision "provides levels and types of criminal offenses that ICE directs to be a focus, rather [than] pursuing immigration enforcement based on specific charges," according to the background documents.