What's the difference between ‘Available to Reserve’ & ‘Available to Transac’ for EAM

Hi, there

        After I release the eAM work order, the material associated to the WO will auot-allocated. And I can find out the number of ‘Available to Reserve’ & ‘Available to Transact’ reduced as hoped(Path: Material  Workbench>Availability).

        But the number of ‘Available to Reserve’ & ‘Available to Transact’ always be the same. What's the difference between ‘Available to Reserve’ & ‘Available to Transac’ for EAM ? When will they be different?

Best regards 

Peter