Normally a sales/transfer order with shipping advice complete will not create an inventory pick if you have insufficient inventory. In this scenario the existing albeit insufficient inventory cannot be reserved for the order via an invt. pick, and may be reserved by other orders with lower stock requirements that are able to create invt. picks.
Back Order Management allows you to reserve the partial inventory for orders with shipping advice complete, by allowing you to create the invt. pick regardless. This invt. pick cannot be posted until it can fully satisfy for the order's shipping advice.
Enabling Allow Partial Inventory Pick
Navigate to Inventory Setup, and enable "Allow Partial Invt. Picks"
Creating Partial Picks
Inventory picks can be created like normal when Allow Partial Invt. Picks is enabled. See Pick Items with Inventory Picks.