If you check this box, you can register a particular account to be used as the accounts payable for this supplier.
You enter general accounts for accounts receivable and accounts payable in the Chart of Accounts / Standard Accounts procedure in Global Settings. These accounts will apply to all customers / suppliers.
You can use several accounts for accounts payable (for example, have a separate account for EU suppliers). Here you can then enter a particular accounts payable account for a particular supplier. This means that the standard accounts payable account will not apply to this supplier.