Temporary duty assignment
Temporary duty travel (TDY), also sometimes referred to as Temporary Additional Duty (TAD) in the US Navy and US Marine Corps, is a duty status designation reflecting a US Government Employee's official travel or assignment at a location other than the employee's permanent duty station. This type of secondment is usually of relatively short duration, typically from two to 189 days in length.
Source: Wikipedia — Temporary duty assignment (CC BY-SA 4.0)