Trust policy clarification

[JuhaFin69]

I need to have some clarification for Signal trust policy and cli usage.

1. what "new identities" for signal-cli means?
2. What is identity key?
3. multiple keys for one number, what means? Does a new identity key be generated only when user changes to a new phone?
4. "--trust-new-identities -always" has same effect that using always "trust -a" parameter?
5. Does I understand right that receiving number becomes untrusted only if user changes his phone?

I try build signal-cli based system that is able to send Signal messages.
I see no alternative than always use parameter trust -a when sending messages, although it is said to be "for testing only". In practice my system has no way to get receiving target phone's safety numbers. I think that ordinary Android/ios app is doing the same thing(trusting unverified numbers). Otherwise regular messaging would not work after people change their phones.