A Circle may delegate responsibilities, tasks and authority to anyone to be carried out autonomously within the limits set by the Circle (examples: social media content creator, bookkeeper etc.)
Work roles can be filled by more than one person but there is only ever one lead per role. The operational lead is responsible for ensuring that the responsibilities relating to that role get done. Circles are responsible for creating and changing their associated roles.
Work Roles are created and changed regularly and a full list can be found at in the Roles folder of the Everyone folder in Drive.