When you customize per account, every account starts as a copy of the post and you change only what that network needs: a shorter text for X or Bluesky, another picture, its own first comment, its own thread. Threads now work on Bluesky too, posted as a chain of replies once the post is live. A Preview button shows how each account will show the post: the text folded or cut where that network folds or cuts it, the pictures, the thread, the first comment. And editing a draft brings its per-account copies back (they used to be lost).