I don't even know what it is.
They are cities where there is a policy intended to limit local law enforcement helping federal officials to enforce immigration rules, the details differing widely depending on the locality with the policy. They become a target when an illegal immigrant commits a crime against someone in one of these places.
In general, local police should not be enforcing immigration law, and should focus instead on building trust in communities with large numbers of undocumented immigrants. SF may go too far in that it does not hold people for the feds even when there is a previous felony conviction, though it may be that they find this is necessary in general in order to get cooperation. Overall there is no correlation suggesting that sanctuary city policies lead to higher crime.
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