There's a get function that returns that. But this is a value that, I am told, is very easy to spoof. For FileMaker Pro users, you might instead use Get ( persistentID ) instead. But with the upcoming iOS7 there's a known issue and we are all waiting to see what FileMaker does about it when this version of the iOS leaves Beta status.
For More Information see: http://forums.filemaker.com/posts/1ebde0f441?commentId=249274#249274
This is one of many acknowledged bugs that can be found in the Known Buglist for Filemaker GO thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip