What is the difference between membership and securables? Does one override the other?
When setting permissions for a database user, what is the difference between 'Membership' and 'Securables'?


Membership is what the user belongs to... Since users are a database principal this could be database level roles, application roles, etc.

Securables are what the name implies, items that are authorized access to.

So, let's say we have a login which is a server level principal. The login can belong to a server level role.. it can be a member.. of that role. Then we give the server level role access to server level securables, such as and endpoint. Maybe we grant control endpoint to this role. Since the login has membership in this role it is granted the control endpoint permission against the endpoint securable.