Hi Raju,
Account assignment reference is used as a criterion for account determination. In Account determination, where GL account is assigned to account symbol, you will see a column for account assignment reference; so different GL accounts can be assigned to different account assignment references. Account assignment reference is determined by derivation rules which is quite simple based on product type and transaction types.
To see what was the account assignment reference derived for a transaction, display the transaction and go in Environment-Positon Indicator.
You may assign various positions G/L to these account assignment references, eg one for FD one for commercial papers. The way you want to see your balance sheet posiiton that way you can define to which GL to post.
Regards,
Jyoti