When capturing goods in order module you have to understand that there are 3 different possibilities that can occur.
a) You have enough inventory to cover the order quantity – In this case your only option is to capture the goods form inventory. Capturing the good will automatically subtract the amount from the inventory.
b) You have inventory, BUT not enough to cover the full order – You have 2 options,
a. Partial Capture: You can capture the quantity available form inventory and create a PO for the remaining
b. Full Capture: You can create a PO for the full amount and leave the inventory as it is
c) You don’t have inventory for that good – You have to create a PO to request the full amount of the order