My best guess would be minimum departure fuel required before take-off.
Situations such as weather ground holds with engines running might take an aircraft below departure fuel levels.
I've never seen a crew make a log book entry on departure fuel. They just verify before push-back that enough fuel is available.
If they're sitting for a length of time before take-off, they'll monitor the fuel quantity. If it falls below allowable..... they'll return and get more.
