In the case of WD Get(PersistentID) does not represent the device but the session:
Returns a unique, unchanging identifier for the computer on which FileMaker Pro is running, the device on which FileMaker Go is running, or the current FileMaker WebDirect session in the form of a 32-digit hexadecimal string. Get(PersistentID) helps you identify devices that access your solution
Thanks much Colin, I missed that. I should have done a simple search.