Appointment Shells are a Calendar option that opens a view of the types of Appointment Slots that are available. The User can Add/Modify/View Concurrent Appointments and identify the number of Appointments and duration. The Appointment Shells reflect your Slots, Build, and Rules Setup.
In the example below, Slot Type, Follow Up X 1 Slot, has 2 Maximum Concurrent Appointments. The Appointment Types that can be scheduled in the Slot are Block, Lab, Follow up, and Lab Follow up.

A Scheduling Rule was created for the Follow Up Slot to only allow six AM Appointments.
Appointment Rules will override the Slot Setup. For example, Even though there are eight Follow Up Morning Appointment Slots on the Calendar, the Appointment Rule only allows six Appointments in the morning.