What does it mean to come out as trans

Does coming out as trans mean someone who is currently presenting as their birth gender tells someone that they feel more comfortable as the other gender and then begin to transition?

Or is it someone who is already post transition and passing telling you that they were born the other gender.

Cause I keep seeing in twitter posts about people coming out as trans to their partner and I’m so confused what that means and the discourse around it is confusing me.