Dear Raju,
AAR 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.
Please refer below link for better understanding.
Account Assignment Reference is Missing or Undetermined - ERP Financials - SCN Wiki
Regards,
Jain