A sales order is eligible to be split if the sales order contains at least one fulfilled item and one that is not fulfilled. The sales order must also have a payment line and in be "pending" status.
1. First, open the Payment
window and click Split
2. This will take you to the "Delivered" split SO, which contains the fulfilled items from the master so. Click the "?" at the bottom of the page to see the master SO.
4. The "Master SO" link will take you to the "Pending" master SO, which contains the un-fulfilled part(s).
Note: Reversing a split sales order will put the fulfilled quantities back on the master sales order