Rollup By Lookup (RBL) is a feature in Financial Services Cloud that allows you to display summary calculations of financial account information, such as balances, market values, and cost bases. You can use RBL to roll up financial data to all joint owners' primary households by doing the following:
Create custom RBL rules to rollup the joint owner balances using the Joint Owner field on the Financial Account object.
Enable RBL rules that rollup balances using Financial Account Role for joint owners using the Account Role field on the Account Contact Relationship object. Verified Reference: : Salesforce Financial Services Cloud Implementation Guide, page 75.