Need document to implement Subscription User access setup with respect to specific BU

Summary:

 Need document to implement Subscription User access setup with respect to specific BU.

Content (required):

Need document/note to implement Subscription User access setup to restrict specific user to particular BU, as well as how to have single user added to multiple BU's.

How to address, if we have a super users, who needs to have access to multiple BU's.

Version (include the version you are using, if applicable):

23c

Code Snippet (add any code snippets that support your topic, if applicable):