When you open a map, PTMapper creates a .lck file. This is a lock file and it will have the same name as the map that you opened (e.g Mymap.mpd and mymap.lck).
This file is created to protect the map file while you are working on the map. Should another user open the same map while you have it open then they will receive a message saying that the map is “already open or is a read-only file”:
PTMapper also indicates this by showing the map’s tab as a yellow box (instead of the usual red one):
The user who opens the map after you will not be able to use Save to save the map, as it will not be available.
Instead, they can either wait for you to close the map or they must use Save As.