Berth got a reaction from Simon in What does the number mean on a MOC inventory?
Its the size of the part. It's usefull when you can't count the studs. It's the same for cross axles or dishes for example.
EDIT : Thanks to that I know that there is 10 1x4, 1x6 and 1x8 tiles and 53 2x2 tiles. I don't need to hover each part.