see

https://docs.servicenow.com/bundle/vancouver-platform-administration/page/build/platform-functions/concept/platform-support-functions.html#:~:text=Function%20fields%20do%20not%20have,%2C%20reports%2C%20and%20so%20on.

Create function fields and scripts in the Now Platform to perform common database transformations and calculations.

Regular fields store a value in the database. Instead of storing data, a function field displays the results of a database query. Function fields do not have an associated database column. Instead, function fields generate a value based on simple computations of other fields and constants. They can be used like any other fields in the system: in forms, lists, query conditions, reports, and so on.

Warning: A function field that references another function field in the function will result in an error. Function fields don't hold data or evaluate in run-time.

Example: Use case

You want to identify all incidents with a probably useless short description of fewer than 10 characters.

Instead of querying all records to determine which record meets a given criteria, create a function field or function script that retrieves only the records that meet the criteria.