Change of doc type in billing type

in billing document type F2, document type is assigned as RV but in billing document type S1 no document is assigned (it is blank). during cancellation of billing document, finance voucher is created with document type AB (which is not assigned in S1).

i want to change document type in S1.

can i assign new document type (self generated) or should it be RV ?

also from where it picks AB as default ?

kindly provide me solution.

warm regards.

Manjit