What is app-centric. Mastodon is one example, but all increments to 'what constitutes fedi' come from app development, where post-facto interoperability takes place.
So Peertube at the time they extended AP were the first to do so in a Video domain, and could freely define AP types and props. Choose them for best-fit with their app.
The next Video app builder then has a choice. "Do I follow that? But it is so peertube specific. Do I want to include their namespace in all my messages and then add my own app-specific namespace for the new props I introduce that maps to my app's features best?"
This is a Grassroots standardization process thingy where ideally..
- Peertube being first is free to extend.
- When the next app dev comes in that business / application domain we weigh the impact, perhaps conclude no coordination is needed yet.
- When more apps come, there's need to formulate a standardize approach. Consolidate protocol decay, and rebalance specs with wire reality.