To populate custom fields on the User record during SSO, Salesforce expects the architect to use the SAML Just-in-Time handler pattern. That means implementing the appropriate JIT interface and supplying create and update methods that map incoming assertion data into Salesforce user attributes. Session management classes and generic registration-handler interfaces solve different problems. The JIT handler exists specifically so that user creation and user updates can occur at login time based on federation data. This is useful when the identity provider is the source of truth for workforce attributes and those values must be refreshed without manual administration. In short, the interface gives Salesforce the entry point, and the create/update methods carry out the field population logic. This is why options A, C work together as the correct solution.