It is possible to define a set of rules to control the format of the End Entity fields. For example, it is possible to ensure that the subject DN serial number is always a number of six digits, or should always end with the letter N.
Setting such rules is done by implementing the static function org.ejbca.core.model.ra.FieldValidator.validate(). In this function, you can define a rule that is applicable to a specific field in a specific End Entity profile. Should the field value not match the rule, a CustomFieldException should be thrown and the error message you set in the exception will be shown as the error message in the GUI. This rule will be checked each time an end entity is added or changed, whether it was added or changed by the GUI or otherwise.
To avoid losing these rules when updating the EJBCA version, the new FieldValidator class should be stored in an ejbca-custom folder. See the Handling changes in a separate tree page.