The strip under the composer shows the repository the discussion is working in, its branch, and what has changed.
The button offers the next step
Work that is written but not committed offers Commit. A commit that has not left the machine offers Push. A pushed branch offers Create PR. An open pull request offers View PR. The button is whatever comes next, not a permanently greyed-out one for the thing you cannot do yet.
The caret beside it holds everything: Stage All (git add . on its own), Commit, Push, Compare Branch, Checks, Merge, Create PR, Create Draft PR and Compare on GitHub. Anything unavailable says why on hover rather than disappearing.
Pull requests
Create PR commits what is uncommitted, pushes the branch and opens the pull request in one go, with the lead's closing word as the body. Create Draft PR does the same but marks it draft — with a room working unattended, "finished but not read by me yet" is the common state, and a draft is how you say so.
You do not need a session branch. Working on a feature branch in your own checkout opens a pull request perfectly well; the only thing that stops one is sitting on the trunk, and there, if your work is still uncommitted, the app makes the branch and carries the changes onto it, leaving the trunk where it was.
Pull requests go through the gh command line, so it borrows the identity you already authenticated — the app holds no GitHub token of its own. Enterprise hosts and SSO work because gh already solved them.
Reading the change
Compare Branch diffs the branch against its base in the app, computed locally, so it works before anything is pushed and before there is a remote at all. Every file the room wrote is also listed with a diff as it goes.