If the trade occurs at or above the ask price, it is considered a buy (ask volume). If the trade occurs at or below the bid price, it is considered a sell (bid volume).
If a trade occurs at a price between
the bid and ask price, then by default, the following logic is used.
If price just moved up on this trade or the previous trade, it's
considered a buy. If price just moved down on this trade or the
previous trade, it's considered a sell. Otherwise, the trade is
considered neutral from a delta perspective and not placed in either buy
or sell volume.