You can see the available balance for a part after that the entered quantity on the order row has been deducted. This is configured by the setting Show available balance on order rows in Register Stock Order? under Stock order under the Inventory tab in the Settings procedure. The available balance is calculated according to the alternative that has been entered with the above mentioned setting.
If the available balance is crossed out it means that the part is not stock updated or that the part is a Bucket part. The special case Bucket part with stock update is also shown with the available balance crossed out.
The available balance that is displayed is an instantaneous value of the order window at present. If e.g. anybody else performs a clearance of an order row for the same part while this order row is being registered, the available balance is actually lower than what is displayed here.