JobTracker tracks inventory based on what you order, receive, and allocate to jobs. The following counts are calculated:
On Hand Qty - quantity in stock whether or not it has been allocated
On Order Qty - quantity on POs that have not been received
Allocated Qty - quantity allocated to jobs
Available Qty - (On Hand + On Order) - Allocated Qty
If you are just getting started with inventory download the Implementation Guide for Material Management.