Sometimes they have a temporary need for pilots from one base to work another base for a month. TDY is an expression of filling that need. At Envoy you have involuntary TDY, voluntary TDA, and Home Based (usually called Home Based TDY). Different rules apply to each version. None of this is really relevant to your original question though, except that Home Based TDY also uses A12 passes as though they're positive space travel.

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Sorry for the derail.