get value for SaveGameObject UserIndex?

From what I could figure out, the SaveGameObject::UserIndex member is supposed to be used on certain platforms that don’t really let people log into “real” seperate user accounts (e.g. the user selection on some consoles, that still gives you access to the other users’ games etc) to select which user the save game belongs to.

My question is now: How do I get the value to provide in there? Will it be auto-initialized when creating the SaveGameObject instance? Or do I have to use some platform-specific logic to query it? or is the information i was told simply wrong and it’s more of a “savegame slot” thing (even though I’m pretty sure SaveSlotName would be used for that)?