Scope of built-in module map

[littledan]

In the specification draft, algorithms which deal with built-in modules accept a referrer parameter. I'm trying to understand how this would be controlled in practice. Naively, I was imagining that the built-in module map would instead be referrer-independent and shared Realm-wide.

I don't quite understand how a host would make another kind of scheme work, as the BuiltinModule.export API doesn't give you any particular parameter in its signature to modulate how widely it takes effect. Some random thoughts:

• Are we anticipating that there will be host-specific APIs similar to BuiltinModule.export which give more explicit scope control?
• Would the scope be a single module/script by default, and you'd concatenate the built-in module polyfilling to the beginning of each JS file?

In terms of spec mechanics, I see scriptContext frequently referenced when calling these host APIs. If someone could explain what that is supposed to refer to, then that may clear up this question.