I'm not sure I fully understand?
https://sandboxcernercentral.com/ is solely for creating a system account for system access, if your app is being launched within the EHR or has a user actively performing an action that results in FHIR calls being made, then you will want to use provider access. If your app does not have a user performing an action which results in FHIR calls being made and a
system is executing calls on it's own, maybe through some automated process like a cron job, then you'd use system access.
Let me know if you need further clarification or if you have other questions.