OBYC--Difference Vax & Vay

Dear Gurus,

Can you pls tell me the difference between Vax and Vay.In SAP help its given that:

VAX: for goods issues for sales orders without account assignment object (the account is not a cost element)

VAY: for goods issues for sales orders with account assignment object (account is a cost element)

Please can any one expalin me with example.I have checked in my present system for SFgoods Vax--is created under misc income account group and Vay is created as clearing account.

But for Finished Goods both are Clearing account.Actually thaere are some new materials for whicl val class needs to be defined I am unable to decide on the GLs for Vax and vay.

Pls help.

Regards

Raju