I think it means that the diff should be filled when cold.
The rear looks easy to get at, but the front is going to need a couple of underbody plates removing by the looks of it.
Going to check the propshaft uj's whilst there, definitely picked up a hum from the rear end on the motorway - it is on MT's which do tend to be noisier, but this sounds like a recent addition !
Cheers
Pete