If you have the same compiled or compiled+merged application running on 2 or more computers and sharing a common datafile via file sharing, only the first machine that opens the datafile will have WRITE access to it. Subsequent machines that try to access the datafile will only be able to open the datafile, but will not be able to make modifications to it. That is, they will have READ ONLY access. The machines with READ ONLY access can open the datafile and may seem able to modify the data, but any changes made on these machines will exist only in the database cache of the individual machine and can never be saved back to the actual datafile. Therefore, it seems that everyone can access and change the datafile, but in reality all modifications made by machines with READ ONLY access are never saved to the datafile.
NOTE: If you wish to be able to have multiple users access one datafile and make changes to it, you need 4D Server.