Accounts Payable AAI's

Hi

I am very confused about the Accounts Payable AAI's

Item PB - Default Bank Account

Item PC - Offset or Trade Account

should both of these items use the same account

example

Current setup

PB  is setup to have account 100.3221.ABCD : which is ABCD Bank clearing account

PC is setup to have account 100.3211 which is trade payable

Once a voucher is entered

                                             DR.      EXPENSE ACCOUNT

                                                       CR. BANK CLEARING ACCOUNT

Once the Voucher is posted

                                              an offset document type AE is created with CR. Trade Payables

the question is

How is the bank clearing account is closed

Thanks