Unique identifier assigned to the EXE when it is built.
First available in version 3.0 build 2039.
The GUID, or Globally Unique IDentifier, is a specially formatted string of hyphenated hexadecimal characters. The exact value is selected randomly by SWF Studio when you build your application or screensaver, but the hyphens are always in the same place. An example of a GUID is "9B4DAE02-CB31-4480-9F43-CD307EFE3187".
Because of the astronomically large number of permutations possible for a GUID (3.5 * 10^38), it is extremely unlikely that a value will ever occur twice.
Anecdotally, to get a sense of how big that number is, we can compare it to the estimated number of stars in the universe. It is thought that there are about 400 billion stars in our galaxy, the Milky Way, and that there are around 80 billion galaxies in the universe. That would make for 3.2 * 10^22 stars in the universe. The number of GUID permutations, 3.5 * 10^38, is more than 10 million billion times larger.
This variable is set when SWF Studio starts, and whenever App.updateGlobals is called.