Azure RBAC roles can be assigned to users, groups, or service principals. A role assignment grants permissions on a predefined set of resources called "scope".

The widest scopes a role can be assigned to are:

* Subscription: a role assigned with this scope grants access to all resources of this Subscription. 
* Management Group: a scope assigned with this scope grants access to all resources of all the Subscriptions in this Management Group.

In case of security incidents involving a compromised identity (user, group, or service principal), limiting its role assignment to the narrowest scope possible helps separate duties and limits what resources are at risk.
