All -- A tip of the hat to "Deer smearer" for post #11.
-- In the case of an RCA volunteering to work at another post office, there is the USPS 05 APR 1999 letter from Charles Baker.
-- For that to happen, manglement has to ask the regular carrier on the RCA's assigned route if he/she plans on taking leave during the time the RCA would be volunteering at another office.
-- If yes, regular submits a leave request, making the RCA unavailable to the other office.
-- "If no, the RCA may be loaned to the other office for the day(s) in question and will be unavailable to replace the regular carrier in the home office for scheduled leave."
-- Maybe the USPS is relying on the "If no" portion while overlooking the "volunteering" part by the RCA.
-- In a bit of a stretch -- Article 30.2.C.2: When necessary or desirable, a substitute, rural carrier associate, or a rural carrier relief employee who is assigned to one route MAY BE UTILIZED on up to three routes. However, the employee's prime responsibility is to the assigned route. ( the USPS's out could be that no office is specifically noted - as I said, a bit of a stretch, not that the USPS is above "stretching" from time to time. )
PS -- Here is something from the "APO LLV - RMPO POV new requirement for RCA'S?" thread in the General Discussion ( around March 2018, or so):
Since I started, RCR's, RCA's TRC's never were required to work outside their assigned office. Then came Post Plan and then RCA's could be required to work in APO & RMPO. Is it really a stretch to foresee the next step down that road to make RCA's work at other offices and interpreting a qualified RCA as one above the dirt with a pulse. The Clerk Craft must go to other offices when needed, so I suppose the USPS will hang their hat on that little detail if this ends up at arbitration.