1) The app must be fully open source: backend, frontend and everything required for an independent self-hosted work;
2) The app must implement at least one of the protocols that allow it to interoperate with other federated networks: OStatus, diaspora protocol, Zot, ActivityPub;
3) If in the future other federating protocols appear, the scope of the website may be expanded. However, interoperability is key. An XMPP, Matrix or any other world of federating apps may be covered only if any of them start federating with existing Fedi networks or if Fedi projects integrate these other protocols. TL;DR: bot-like functionality of reposting from some networks to others is not considered interoperability. Two-way interoperability with support of basic things (limited view messages, likes, reshares, comments) is expected.