Hello Sarath,
As mentioned by Siva in the above the table name is TVAK but if you are a functional guy then you should go by the functional assignment.We are assigning the default billing type in Tcode= VOV8.
All sales document control you can find over here.
Needless to mention this Billing type you are assigning over here is auto proposed during billing in VF01. But you have option to choose another billing type in VF01 itself considering you have maintained the Copy Control for that.
Thanks,