Hello,
The rule BR21 you mentioned will test if the field knb5-gmvdt (date of legal dunning procedure) is initial or not. You can check the help of the field in the customer master for more information.
If the field is not initial, the rule will hit. Technically, It reads the data from table FDM_COLL_ITEMS (updated in batch by program FDM_COLL_SEND_ITEMS which reads KNB5-GMVDT). The basic structure is not necessary here and the rule is functional without it.
If you want to test if the customer is in dunning (check if there is a dunning level on one or more items), you need to use the rule BR00000017.
Regards