smPlaylistOrder is an Alias for the number 8.
i.e. instead of:
You can also write this:
Cool, isn't it?
OK jokes aside: smPlaylistOrder or the number 8 for that matter can be used together with stuff like CAT['xyz'].ChooseSong, CAT['xyz'].QueueBottom, CAT['xyz'].QueueTop
It tells each of these commands to pick the next song according to the custom order by which you sorted your category xyz. The song at the top will be picked first, then the one below that and so on. (Obviously it will skip songs that don't match your repeat rules if you tell it to EnforceRules)
They use a constant for that purpose because that is easier to remember than putting 8 in there.
It's similar to the InsertPosition for QueueDir for example (ipTop = 0, ipBottom = 1)