FSN indeed, FGO currently only has alternate versions of her playable/involved in plot (different characters possessing her body, tagged as Ishtar and Ereshkigal) though this outfit https://fategrandorder.fandom.com/wiki/Street_Choco-Maid being exclusive to FGO artwork has created a few edgecases where FGO can be the correct parody tag for Rin on galleries like /g/1696423/78c3ed69e9/ this.
FGO is starting to become a generic series tag for Fate in general because it features almost every character in the franchise (Rin only being half of an exception, because characters like Kama, Muramasa and the afforementioned Ishtar and Ereshkigal are definitely designed that way to sell the original FSN characters to people while technically still new characters) for the ones that do overlap setting/context can narrow down the parody tag. But in the Cosplay category we don't get any of that and the conflict is only really resolved when an outfit is parody-exclusive if the character itself isn't.
@Byza I just categorize it as Fate SERIES in my computer, there are too many crossovers to keep track of. Just like the Toaru SERIES for index and railgun. And the Idolm@ster SERIES and the Love Live SERIES.