what is the significance of this trn code?
i have seen that after picking and PGI ..without going for lt03 am able to create billing at vf01...so why do we need LT03???
LT03 is used for creating transfer order . If you are actived Warehouse management, the picking filed will be grayed out in Delivery (no maual picking allowed). So you need to create a transfer order using LT03 and confirm the transfer order using LT12. Once the tranfer order is confirmed, the pick quantity will get updated in the delivery and you will be able to do the PGI.
Since you have not activated WM in your structure, there is no need or trnafer order to pick the material, youm may manually enter pick quantity in delivery and do PGI.