Comparing it to the numbers from coinmarketcap.com, which specifies units and breaks down the volume into market places and currency pairs, yields the following results:
- The volume given by Yahoo has to be in USD. If interpreted as BTC, the number would be far too high.
- The volume only includes real BTC/USD trading, not BTC/USDT (using Tether, a pseudo-$ on a blockchain, in order to move classic fiat money easily between exchanges).
- The volume excludes markets that don't charge trading fees, otherwise the volume would be far higher.
Summing up all 24h market volumes of the BTC/USD trading pairs without fees gives a result in $ that is similar in magnitude to the number given by Yahoo as "Volume (24h)". Since the number on Yahoo is slightly lower, I guess that they don't cover smaller exchanges, only the biggest 6 or 7, approximately.
The number "Volume" is the volume traded today (resetting at midnight UTC), the number "Volume (24h)" is the volume traded during the last 24h.