My guess would be Royal Engineers Depot. This is/was a unit based in Chatham to which soldiers were posted on paper between postings to field units or during a long course or prior to, for example, discharge. A soldier must be held a unit strength at all times during his service with the colours - the regular army part off his commitment - and so the depot is a convenient unit to hold soldiers in transit etc. He could get his pay and allowances sorted out, apply for leave and travel warrants etc through the depot without necessarily being accommodated there or having any duties there.
