The area settings for the Membership import allow you to determine how to deal with multiple memberships on a record and the match criteria.
In the case of membership, the RE business rules can prevent you from having multiple memberships. In the case above this is the reason that the “Always add a new membership” radio button is disabled.
Where multiple memberships are allowed, you can set the match criteria. It is possible that there will nevertheless be multiple matches made. In this case the priority order is used to determine which membership should be updated.