Where volume exists and where it does not
Crypto pairs have real exchange volume. Forex has no central exchange, and "volume" there is the tick count of a particular data provider. That is why the volume factors in the set are marked as crypto factors: on forex they are either not counted or counted on a proxy.
OBV
Cumulative volume signed by direction: it rises when the close is above the previous one and falls when below. The meaning lies in divergence: if price makes a new high and OBV does not, the move is not backed.
MFI
The same RSI, weighted by volume. The zones are the same (20 and 80), but an overbought MFI means not merely a rising price but a rise on large buying volume.
CMF
Chaikin money flow: where inside the candle price closed, weighted by volume. It counts as a factor when the flow runs against price — a sign of hidden resistance to the move.
VWAP
The volume-weighted average price since the start of the period. In practice it is the "fair price of the day". A stretch of more than 1.5 ATR away from VWAP in a range counts as a reversal factor: price has moved too far from the level around which trading took place.
Volume climax
A volume spike noticeably above the usual together with a long candle. It often marks the exhaustion of a move — the last buyers are already in.