FieldEdit PeopleCode fires for the field and the row that just changed.
So, to validate the input entered by a user into a field, you may write PeopleCode in the FieldEdit event of that particular field.
This is the event you should be using for warning and error statements to convey appropriate messages to the user.
However, to check for consistency across fields on a page, PeopleSoft recommends using the PeopleCode SaveEdit event.