It might make the spec easier to understand if we could get consensus on what the granularity of a source is.

We know that:

1. at the minimum, the source is shared between cloned tracks: a getUserMedia call is the minimum granularity for a source
2. a UA wide source per device is not ok, device Ids are not shared between origins for instance.
3. A source can also be at the global object level, or at the top level global object level.

The spec in some places may assume 2 or 3.
The most flexible option is probably option 1, this was used like this in the past in some Safari cases (where multiple cameras cannot run at the same time for instance).