If you are using PerfectTablePlan across a network (e.g. 2 or more users accessing the same file on a shared disk) you need to be careful that users don't overwrite each other's changes.
PerfectTablePlan uses Windows file locking to ensure that only one user can open a plan file to write (even across a network). Any number of users can open the same plan file with read-only access. File locking can be enabled/disabled in the 
Preferences window.
