Okay, another attempt. (Let me say, that installing VIS isn't less confusing. )
Valdacil's Item Sorting
I go for the "by class" option but I guess it's the same for the "by slot" one.
From the main mod, v. 9.0.3 – has to be installed first:
ValdacilsItemSorting-ArmorByClass.esp – Name of the file that would sort armor by class for a non-DLC game.
ValdacilsItemSorting-ZZ-DLCArmorByClassOverride.esp – Name of the file with DLC.
The FOMOD tells that only one has to be used, and it should be loaded last. (Well, sort of – the same is said for the cosmetics and weapons ESPs.) I guess, this is why the current LOOT metadata contains the cited load order.
Then there is an update in the files' section, v. 9.1. It requires the DLC. There are no specific installation instructions, no ReadMe, only the file packed for an MM setup. This file's name is – bad choice – ValdacilsItemSorting-ArmorByClass.esp. Which is the same name as for the above listed non-DLC version, but this time with DLC. As it has the same name, LOOT metadata is applied to it. But as it also is an update to the ValdacilsItemSorting-ZZ-DLCArmorByClassOverride.esp, it should be loaded after it, I suppose.
I have to tell that I haven't done a detailed look into the file's contents, I haven't compared the 9.0.3 and the 9.1 versions of the ESP, and it might be that the older file isn't necessary. Disabling that ZZ-DLCArmorByClassOverride-plugin would indeed solve this conflict, but do I know if there are any entries that are still needed?
Okay, seems this is redundant. Searched a bit through the mod's forum, and somewhere there's a hint that ValdacilsItemSorting-ZZ-DLCArmorByClassOverride.esp can be disabled when the update is installed.
Edit: After disabling the redundant (and possibly conflicting ESP) LOOT warns with "You are sorting armor by class and also using DLCs. You may want to use "ValdacilsItemSorting-ZZ-DLCArmorByClassOverride.esp" to enable the class sorting on DLC armor as well.