If all of the inventory on a stock transfer is not yet ready or available when you pick the stock transfer, you can configure Inform to spawn a new stock transfer for the remaining products.
To do so, set File > Company > Master Spawn New Transfer for B/O items after Shipment Confirmation to Y.
When you enter initials in the Picked by field on the Stock Transfer, Inform will spawn a new Stock Transfer for any unpicked items remaining on back order. The new Stock Transfer is created with the same Notes, Warehouses, Writer, and Consolidate settings as the original.