What is the use of this check box in the shipment lines:-- automatic adjustment of quantity not shipped. .

Never used it, but according to the Session Help ...

If this check box is selected, ERP LN creates and processes an automatic adjustment for any quantity not shipped (that is, where the value of the Shipped is less than the value of the Picked in the current session). The unshipped quantity is also removed from the inventory.
If this check box is cleared, and there is an unshipped quantity, the quantity is left on the staging location. You can then transfer that quantity to another location to perform another adjustment, or return it to bulk or pick locations.

Mike