The U.S. Department of Homeland Security (“DHS”) – U.S. Citizenship and Immigration Services (‘USCIS”) does bring removal proceedings against every alien whom it suspects of being deportable. For one thin, it recognizes that there are certain cases in which extraordinary sympathetic factors would make removal unconscionable. For another, the DHS could not remove all deportable alien even if it wanted to. Removal proceedings require apprehension, investigation, proceedings, possible detentions, prosecution, adjudication, removal, and recording-keeping. Like any other government agency, the DHS has limited resources. It has to decide how it can most efficiently allocate those resources (a) between law enforcement functions and other functions, and (b) within law enforcement.
For some time the DHS policy has been to refrain from initiating removal proceedings in certain unusually compassionate cases.That policy has been given different names over the years: “prosecutorial discretion”, “nonpriority status”, and most commonly today, “deferred action”. Whatever the name, the theory has been that the case is simply put on the back burner. Technically the DHS remains free to proceed against the alien in the future if its workload or its priority change, realistically, since the actual reason for holding back is the presence of exceptionally sympathetic factor, relief will typically be permanent unless those individual factors change.