Hi Laura,
In my opinion, risk class is the evaluation of a customer based on the check rules, and based on the evaluated score; the risk class is assigned at master data.
So, let's say, you have assigned a credit check rule to a customer. In SPRO, the credit check rule has been configured with a Score Formula. When you process the score evaluation either from BP screen or using mass processing program, the business partner would be assessed as per the formula. Whatever score has been calculated from the formula would be checked with the risk class mapping, internally during that process and then the appropriate risk class is calculated and assigned at business partner (Table UKMBP_CMS).
At BP level, you can assign a check rule using BP screen and that is stored in table UKMBP_CMS.
And, you can assign ratings internally (FICA Rating from creditworthiness) or externally(from credit bureau). Those ratings are associated with corresponding rating procedure so that you can segregate ratings as per rating procedure. That is stored in the table BP1012 time sliced with ratings.
The overall Score Formula (that you'll build) associated with the check rule assigned at BP, would then evaluate the customer based on the ratings and other factors. The score is then evaluated and stored in table UKMBP_CMS in field OWN_RATING. The program UKM_MASS_UPD2 is used to calculate score at BP that assigns risk class as well.
Hope it addresses your query.
Thanks,
Bodhisattwa