a period of 'special activity' when the aircraft will 'stay' in the area defined for a given length of time.
EUROCAE (2009) FLIGHT OBJECT INTEROPERABILITY SPECIFICATION ED-133, V 1.0 June 2009
A STAY indicator can be used to indicate the time spent in an area (STAY area) by a flight doing special activities (training, air-air refuelling, photographic missions etc.). A STAY indicator shall be inserted in Field 15 of the FPL between the point of entry in the STAY area and the point of exit from the STAY area.
See also IFPS Users Manual, ed. 10.0