Hi Kumar
Let me try to explain you.
Credit Memo with reference to Billing :-
Say you sent 10 CFL tubes to client each Rs 10 .But when it reached to client site client told you that they have received only 9 CFL tubes working. 1 CFL had some fault in it .So you told your client to retrun that CLF to be tested at your plant and send them a replacement or credit Note.
This s Example of Delivery Based Credit Note .Here you have to receive the good Physically .
So your SAP flow bill be
Sale Order --Delivery---PGI---Invoice
with Reference to Invoice Return Order Credit Note Request--Delivery--PGR(Post Good return)--Credit Note
Order Based Credit Note:-
In Same Example you told your customer that you do not want to take the return rather you will give them 10Rs back.
So with Reference to Invoice your created Credit Note Request--Credit Note of 10 Rs.
It is always advisable to Create Credit Note with request to Invoice because Sale Oder can be modified with Order quantity but Invoice can not be modified so you will always get the correct data from Invoice for future references.
Hope this Helps.