The access levels currently are:
This contact - default - a user is able to access their own bookings
All contacts in this client - a user is able to access their own bookings and those of contacts within the same client
All contacts in this client and child client contacts - as above plus contacts within other clients that fall within the same parent company
To have departments within a client with department heads being able to access their specific team's bookings and no other bookers within that client.
To do this you would load the departments as individual clients all with the same parent company which will separate them out as required.
Bookings can be reassigned so that they appear under the correct client, your reporting will be extended to the department as well as parents.