Capabilities/moodle/role:assign

allows a user to assign roles in context. note that to assign a role A to another user, the user doing the assignment has to hold a role B so that role B is permitted to assign role A. This is controlled in the roles allow assign settings.

This is logical because even though a teacher might have moodle/role:assign, in most situations we only want him to be able to assign users the student role, and not the admin role in his own course.