Although the topic of "sanctuary" jurisdictions is much-discussed, the term "sanctuary" has no legal meaning.  State and local governments face a complicated set of choices about how to interact with federal immigration authorities.  Four major policy questions must be considered.

First, states and localities have to decide whether to contribute resources to federal immigration enforcement efforts. Second, they must decide whether to detain noncitizens at the request of federal authorities. Third, jurisdictions can limit the extent to which they share information about noncitizens with the federal government-although every jurisdiction, even "sanctuaries," shares significant amounts of information about noncitizens.  Finally, states and localities may try to limit the extent to which immigration agents can physically access state-controlled properties, like jails and courthouses. 

In each area, there is a spectrum of choices available, from full cooperation to neutrality to active resistance.  This explainer identifies the actions state and local governments have taken, and identifies resources for policymakers who wish to learn more.

Read more here.