Explanation
The functionality that Rollup By Lookup (RBL) can provide for rolling up customer data at the client and group levels are as follows:
An RBL rule displays summary calculations of financial account information, such as account balances, net worth, total assets, total liabilities, and so on1.
When you edit a financial account record or primary group membership, the Rollup By Lookup (RBL) configuration updates the corresponding RBL summaries at the client and group levels1.
Rollup By Lookup (RBL) displays associated records for Financial Accounts, Financial Goals, and Opportunities on the client and group pages1. Verified References: 1

The following activities should be prioritized by the implementation team during planning for designing the Financial Services Cloud implementation:
Prepare for integrations with transactional systems, external data sources, custodians, and any other platforms that the company's business relies on. Integrations are the connections or interactions that allow data or functionality to flow between different systems or platforms. You should prepare for integrations with your existing or new systems or platforms that support your business processes, such as transactional systems, external data sources, custodians, or other platforms. You should also consider the integration methods, tools, requirements, and challenges that you will need to address during the implementation.
Beyond the preconfigured settings, evaluate whether they need advanced customizations. Customizations are the changes or modifications that you make to Salesforce to meet your specific business needs or preferences. You should evaluate whether you need advanced customizations beyond the preconfigured settings that Financial Services Cloud provides out of the box, such as custom objects, fields, formulas, validation rules, workflows, triggers, or components. You should also consider the impact, benefits, and risks of your customizations on your data model, user interface, performance, security, or maintenance.
Review the out-of-the-box capabilities and compare them to the company's current needs. Out-of-the-box capabilities are the features or functions that Financial Services Cloud offers without any customizations or configurations. You should review the out-of-the-box capabilities and compare them to your company's current needs and expectations. You should also identify any gaps or opportunities for improvement that you can address with configurations or customizations during the implementation. Verified Reference: : Salesforce Help Article [12] : Salesforce Help Article [13] : Salesforce Help Article [14]