Explanation
The following considerations should be reviewed before implementing Flow for customer onboarding:
Use Field Level Security on Milestones, which is a setting that controls whether users can view and edit fields on records. The Admin should use Field Level Security to restrict access to the Milestones field on the Flow Template object, which is used to track the progress of customer onboarding flows2.
Flow for Financial Service Cloud is not available for the Individual Object Model, which is an alternative data model that uses the Individual object instead of Person Accounts to store information about people. Flow for Financial Service Cloud only supports Person Accounts as the primary object for customer onboarding flows2. Verified References: 2

Explanation
A Contact-Contact Relationship is a construct in Financial Services Cloud that allows you to model the relationship between two contacts, such as a business contact and a client that is modeled as a person account.
The Reciprocal Role is a field on the Contact-Contact Relationship object that defines how each contact relates to the other. Verified References: : Salesforce Financial Services Cloud Implementation Guide, page 64.