RE: How to reserve items as they come into stock
If you have the A/R parameter 'Reservation' set to 'Automatic', find an item which has 5 in inventory. Create a sales order line for 7 of the item. The 'Autoreservation' dialogue box will open, informing you that AX cannot reserve the full quantity. If you click 'OK', AX will reserve 5 to the order line, and leave 2 'On order'. If you want to, you can create the PO directly from the Sales order, using the 'New' -> 'Purchase order' button on the sales order ribbon bar. On the 'Create purchase order' dialogue box, you can change the 'Quantity' to 2.
If you have the A/R parameter 'Reservation' set to 'Manual', AX will not automatically reserve the 5 for you. On the order line, you have to click Inventory -> Reservation and enter 5 in the 'Reservation' column.