This function returns a new GUID (Globally Unique Identifier) whenever it is called. The usages for GUIDs are discussed in various places, a starting point would be this Wikipedia article.
An example of usage in XpressDox would be the following, which sets the value of a data element named Id to a new GUID when the user tabs out of the Name data element on the interview:
«Name»
«OnExitSet(Name,Id,Value,,Guid(),,OnlyWhenEmpty)»