Gary,
Available Capacity:
Capacities is driven from the work centre settings in the Capacities tab (specifically the button at the bottom of this tab). These values determine the length of the working day, breaks, number of workers and efficiency - resulting in the daily amount of hours available at the work centre.
Capacity Usage:
Order operations are then created and loaded with work centre and hours, and dependent upon the operation control key settings, will then start to "consume" some of the above daily capacity.
Here is a comprehensive PDF document on the subject
Hope this helps..
PeteA