unique ID Element: Unique identifier (ID).
unique ID string
A unique ID is used by numerous items within the engine. Unique IDs tend to be used where the engine needs to index a set of items. In this situation, each item includes an index and the engine refers to the item by that unique ID.
unique ID is designed to hold an arbitrary string but the engine will generally use CoreFoundation Universally Unique Identifiers (CFUUID).
If no unique ID is provided, the engine can create a CFUUID as a default.