Field help (BSIPYACT) 

Enter a dedicated payment attribute for the suspense account.You can only enter a payment attribute with an accounting sign Unspecified. Payment attributes with accounting signs Expense or Revenue are not allowed. The reason for this restriction is that posting to suspense accounts can be revenue or expense and you cannot enter a payment attribute during a bank statement import life cycle. If the accounting sign is not Unspecified, you receive a blocking message.
The only account structure allowed for the payment attribute is BANK <-> BP. Otherwise, you receive a blocking message.