[Proposal 0002] Clarify default labeler semantics

[austinlparker]

Users can have more than one Labeling Service, with a default given by your server and possibly more suggested as options. Labeling Services can be used by any user and can cover the entire network so it’s not solely the job of a server admin to moderate their space.

It would be helpful to clearly define this concept, along with the word 'server'. The rough questions I have are:

• Is the 'server' a BGS, or a PDS?
• Is a subscription to the default labeler a requirement, or can it be disabled?
• In the event that I disable all labelers, is there a class of content (i.e. CSAM or other legally-actionable content) that will be filtered?
• If the default label service is a requirement, where is this requirement enforced - at the client, at the PDS, or at the BGS?

To editorialize - I suspect that some confusion over how federated labels will work is due to the lack of clarity about how labelers will apply in practice. Supposing that label services are required by the BGS (i.e., bsky.app), if I disable them in my client then I would see all content and accounts with no restrictions, filters, etc., even if those labels are applied to my account/posts, but I would have no way to know that I was being labeled nor would disabling this labeler impact how other people might see my content, which seems like a bad UX. If they're required by the PDS, and I move to another PDS that doesn't subscribe to a default labeler (or to a very permissive one), how then would this interact with the BGS? Would there be a requirement to be indexed by a given BGS that you use its labeling service(s)? How would this requirement be enforced?