But the distinction between source and destination is relevant with some algorithms (one-way sync) and irrelevant in others.
Let say you have two directories A and B to sync and both contain a file f but the one in B is more recent than the one i A. A consistent strategy would be to always favor the most recent file and end up with A and B containing the same f file that was the one being in B at the beginning of the syncing. At no point I asked the user to define a source nor a destination but still the two directories are synced.
Note that it is your choice to continue or not the discussion as much as it was mine (not yours, shockingly) to answer your post. So please keep your "End, full stop" and "Think before answering" to yourself, they are quiet annoying.