Should Cancellation Billing document type be assigned a accounting document type?

We have cancellation document type, that is defined as a cancellation of other regular billing document types. I noticed that custom accounting document type was assigned as part of our customization (from long back).

I am trying to figure out the reason for assigning the account doc. type to cancellation billing doc. type. As far as I can see, the accounting doc. associated with the cancellation doc does not use the 'Accounting doc. type' that is specified (it uses the Reverse doc. type from OBA7 for the associated billing doc type).

For example, Doc 'A' of type ZF2 has Acct doc of type 'RV' and Doc 'B' of type 'ZF2C' has acct. doc of type 'AB' (which is the reversal doc type for 'RV'), eventhough acct. doc type for ZF2C (tvfk-blart) is 'YC').

What is std. SAP practice? should the accounting doc type (TVFK-BLART) be blank for cancellation documents? or it does not matter.