Delivery document types

Hi,

I have doubt regarding delivery document types.

Let me explain in detail,

In Delivery document if we maintain delivery document type "T" it will show "post goods receipt"(PGR) at the time of delivery and if we maintain "J " then it will show "post goods issue"(PGI).

PGI is the last step in delivery process where ownership of the goods is transferred to customer.

PGR takes place when customer return goods to plant.

I have created my own sales document as AOR (copied from OR) and created delivery type as LFY(copied from standard LF) and assigned it to AOR.

I have created normal order with my sales document AOR with out changing delivery document type(it is T in LFY ) and did delivery with respect to the order and my stock reduced form unrestricted stock.

And I changed the delivery document type in "J" in my own delivery type LFY and created order and did delivery, where I found PGR at the time of delivery.

In both the cases stock is reduced from unrestricted stock I did not find stock in returns.

QUESTION:If stock is reduced from unrestricted stock in both the cases what is the purpose of changing document types into T or J.

(I am a beginner so i my question is lengthy as i want learn in detail)

Regards

Bhupathi.