External Identity and self-service onboarding can be combined so a guest begins registration without immediately being tied to a permanent contact record. In this design, the site uses the configurable self-registration page and enables new customers or partners to self-register. The registration handler can be customized so the initial user record is created according to the temporary onboarding model the architect wants. After the onboarding process is complete, the flow can create the full account and contact structure and update the user linkage appropriately. The key architectural point is that configurable self-registration gives the team control over how and when the external identity is materialized in Salesforce, instead of forcing the final business relationship to exist at the first screen. This is why options A, B, D work together as the correct solution.