The additional key enables you to enhance a customer-specific logic for account determination. You can determine a value for this additional key from the data of the payment item in a business transaction event. This is then included in the account determination.
Example: You maintatin an account in Bank Current Accounts for business partner G that is posted to from a sub-ledger loans and a sub-ledger insurance. From the payment item you can tell from which of the two sub-ledgers the item originates (if the transaction type is the same for postings from both sub-ledgers). With the additional key you can control the postings to different general ledger accounts. This means that one account in BCA for business partner G is sufficient for all sub-ledgers.
Hierarchy 
EA-FINSERV (Software Component) SAP Enterprise Extension Financial Services
IS-B-BCA-PT-GLP (Application Component) G/L transfer
FKBH (Package) Bank Customer Accounts: General Ledger Integration
Basic Data 
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