Field help (BPCGRP) 

Reminder group with which the customer is associated.

The reminder group is used to define a group of management rules that are applicable during the launch of the reminder letters.
The Type of reminder and the Reminder threshold is by default recovered from the reminder group during the creation of the record (but not re-initialized if allocated to a different group of characteristics).
However it is the values from the customer record that prevail over those of the group or the category for these two fields (if they are different ).
A customer that is not attached to a reminder grouped is not considered.