The PRE Field Checks occur before the FCL Field Checks. They are executed after the end-user enters a non-printable character. Unlike FCLs, they execute regardless of whether or not the end-user entered in a new non-nil value.
Here is an example (MAGIC) that converts whatever the end-user enters from lowercase to uppercase:
Here is the same example but for C/S:
Here is an example for MAGIC that automatically exits a screen if the end-user does not enter any value into a field:
Here is that sample example for C/S:
This example will work for both MAGIC and C/S. It pops up the exit prompt if no value is entered: