hi, the thing is that initially it was only Delta Chat, and it had a shared Rust core SDK (before Rust was sexy, it probably was the first serious messenger having a full Rust core)

then the Delta Chat client core moved to a more generic naming "chatmail client core" and the server are "chatmail relays" then Delta Chat is just the official client while ArcaneChat and others exists,

the idea is to have an audited core that can be used by different clients then they are interoperable and it also makes it writing a new chat app super easy because the core handles all the encryption and complex logic while the clients just have to worry about UI

chatmail relays and clients have a somewhat similar philosophy to Nostr's relays in the sense that you can have several relays and replace/migrate away from them etc. and your user ID is your cryptographic fingerprint and not some nickname in some server