This document explains Reserved Stock workflow.
When customer places order for items that includes some Reserve stock.
Enter Reserve stock order as per normal. We would still Allocate the order.
Find Reserve Stock order in sales register and hit Ctrl J to go to Despatch register and Pick Order.
3. Double click on items that customer wants from Reserve stock.
In this example, I have clicked on batch and edited qty to 1.
4. Click on Use Selected.
5. Select Split the Order.
6. Select OK.
This will create 2 order. 1 new order and 1 the reserve stock remaining order.
7. Escape from screen and drill to source on the PICKED order from the Despatch register and add additional items and process as per normal.