To replace, edit or delete an existing file, a write needs a basis: either the revision this session read or wrote last, or an if_match naming the SHA-256 the agent expects the live file to have. SyncNow re-reads the live file just before writing and writes only if it still matches.
Without a basis the answer is BASELINE_REQUIRED; if the file changed since it was read, STALE_CONTEXT; if another writer changed it while the session was open, CHANGED_UNDER_YOU; if it was changed outside SyncNow, OUT_OF_BAND_DRIFT. In every case nothing is written, and the agent re-reads the file, applies its change to the current content and sends it again. A new file is created only if it is still absent, so of two agents creating the same file exactly one succeeds.
Appends, number bumps and timestamp touches need no basis, because they do not replace content. If something was overwritten anyway, file_versions lists the previous contents SyncNow kept and restore_version puts one back.