It's the difference between the platforms (usually the bays) and the sidings that are in between the platforms, and those at the southern end of the station. Really it's called Sheffield Station Sidings for the former and Sheffield Shunt Spur/Holding Sidings for the latter.
It's one of those rare large stations where shunts are still untimed, so you can't tell where units are booked to go to and from just from RTT or similar.
You can add Huddersfield and Leeds to that list (used on Saturday night-Sunday morning). The 68/Mk5s are stabling at Scarborough, with the depot temporarily out of use.
Also depends on your definition of 'stabling' as to whether Glasgow Central should be on the list - it's only used for a set that arrives from Polmadie at 00:35 and then forms the 04:22 departure. It's all contained on one diagram.
Similarly Manchester Airport could be contained depending on your definition, one/two diagrams a day "finish" there at 00:17, before starting there the next day at 00:38 (in reality "going to bed" at York depot around 03:30).
