Government UAS pilot reported an airborne conflict with another aircraft during a training flight.

Narrative

I was the acting Pilot in Command and Instructor Pilot during a training flight in the vicinity of ZZZ between 10500-12500 FT MSL. My student was the sole manipulator of controls. The other four crew members comprised of a Sensor Operator Instructor; Sensor Operator in training; Mission Intelligence Coordinator Instructor and Mission Intelligence Coordinator student.A fuel relief aircraft approached from the rear of our UAS. The TCAS system activated a Resolution Advisory; this autopilot status mode was displayed to the crew. The aircraft remained level and once the chase aircraft switched the transponder to standby as part of formation flight procedures; the Resolution Advisory cleared and the UAS resumed normal autopilot hold mode operation.I filed a report within my company's safety reporting system. During the fact-finding process; it was brought to my attention that use of the TCAS system is prohibited when operating a UAS; unless part of a detect and avoid system. This is in accordance with 'FAA Air Traffic Organization (ATO) UAS Accepted Air Traffic Control (ATC) routings and procedures.