The user is about to (electively) post transactions. Return 0 to abort posting. If you return 0, none of the transactions in the selection will be posted. If you do this, you should display some feedback to the user as to why the posting is not being allowed.
You should avoid at all costs iterating over the selection with a for-loop (as in the bad example below). This will run on the client and be very slow if the network latency is high. If it all possible, try to implement your logic using a search with InteresectSelection.
on AllowPostTransactions(toPost) foreach t in transaction toPost if t.EnteredBy <> Initials Alert("Can't post", "You can only post transactions that you entered") return 0 endif endfor return 1 end