Using User Defined Fields you can assign a specific name to any Expression. The expression can then be accessed throughout the event file.
For example, by default, the User Defined Field DisplayName is defined using this expression:
trim([Title] & "" & [Firstname] & "" & [Lastname])
DisplayName is being used on almost all lists to show that expression. If you do decide to use a different expression such as
UCase([Lastname]) & "" & [Firstname]
then you only have to change the User Defined Field once instead of modifying every single list.