Define a pattern (regular expression) to check whether the job name is valid or not.

Forcing the check on existing jobs, will allow you to enforce a naming convention on existing jobs - e.g. even if the user does not change the name, it will be validated with the given pattern at every submit and no updates can be made until the name confirms.
This option does not affect the execution of jobs with non-compliant names. It just controls the validation process when saving job configurations.