LTI 1.3 Registration vs Deployment in edX Instances and IDs

In LTI 1.3, there is the concept of an issuer and platform instance. What is the concept of an instance within edX? (e.g. Is an institution considered an instance?) From observation, edX uses the same issuer URL for all instances?

My questions stem from scoping IDs to avoid collisions? What can we scope as globally unique - sub, client id, deployment id? Could we probably use just issuer as the ID namespace? If not, we will be storing users and classes multiple times (with the same IDs) because they are associated to different registrations (for every assignment).