Any reason why user ID is defined as a integer and global identifier as a string?

[tobie]

It seems both of these are abstract concepts and don't need to be defined as data types.

[jyasskin]

I wanted to be clear that user IDs don't need to be related to any particular real-world identifier and can be minted by any site at basically any time. It's true that a site can use a string as a user ID instead.

I think you're right that we shouldn't give "global identifier" a concrete type.