Suppose a Navy JO in a Joint unit is getting a FITREP from a senior officer from another service. Does RSCA even matter? How would the Navy (PERS) even find out the RSCA, and how would the other service learn about the Navy JO’s FITREP?
Normally, the RSCA goes on the summary letter that is sent with your FITREP. Presumably, the "other service" RS is still doing a Navy FITREP for you, so then, by instruction, the summary letter should be included with the number.
As for the other service knowing about what that RS gave you...<shrug>