in zssapp, CollaborationInfoEx keeps track of the current users who are opening a file. It will remove the relation when a user closes a file or a browser tab.
But the desktop removal request is not reliably sent every time since a browser might crash or too busy to send. Hence, zssapp can't keep track of the real usage of a file, and this will cause an inconsistent state between clients and the server, which is no one is opening the file, but the zssapp still contains users are using that file.
ZSSAPP prevents you from deleting a file