Hi...it can vary per source. But in general the accumulated volume field will be based on that days trading for an exchange/venue and then cleared out and stored in our timeseries database at the end of the market day. So its typically accumulated over a trading day and not dependent on a calendar day if the trading day happens to cross over the UTC date boundary.
In addition to @tim.andrew.smith's answer above, exhaustive (and exhausting) documentation for which exchanges have the ACVOL_UNS field cleared when a closing run update is sent can be found by traversing documentation at https://refinitiv.fixspec.com/stack/specs/elektron
FWIW, the behaviour was clearing on closing run update for all of the many exchanges I was interested in.