To summarize, you have defined scopes to say which objects a role can access and update, a role to say which cmdlets and parameters are made available to the holders of the role, and role groups to help manage who can do what. Role assignments provide what Microsoft refers to as the glue for RBAC in that they link roles and their scope to users or groups. To see the complete set of role assignments in the organization, type:
Exchange 2013 includes some hundreds of role assignments ordered by assignee name. All the roles assigned to the Organization Management role group are found together, as are all the roles assigned to the other role groups such as Hygiene Management, Discovery Management, Move Mailboxes, ...