Kind of a silly question.
I learned about CollectionService early on in my development, and I could have sworn that in the documentation it said that GetTagged and the instance add/remove event functions would only find instances tagged in Workspace. Today I realized that I had a bug because an instance in ReplicatedStorage was being tracked successfully.
Has the scope of CollectionService always been the entire game, and early dev me made a silly mistake?
No question is silly.
In my case, every time I used it, it always returned the instances that have that tag, no matter where they are.
Reading the documentation a bit more, it refers to anything that is a descendant of the data model and gives Workspace and Lighting services as examples, so maybe it was a misinterpretation on your part.
Anything with a tag is considered a valid object to CollectionService. Even objects in places that scripts cant actually access (Roblox might’ve fixed this), and the nil space.