Hi,
Availability check will happen when sales order is created via Idoc. So in that case delivery date to customer can change. So for that SAP should send one Order acknowledge Idoc which will have the exact dates about the deliveries.
Some of the external system provides the real time availability check as well. In that case they do the RFC call and sends information to SAP which will do the availability check by BAPI and sends the result to the external system.
Material Determination will also happen when sales order will be created in SAP. But material determination will work when you will send the information in the correct segment and qualif.
Regards
Suneet
