Inappropriate exchange rate getting picked in billing document
I have a client requirement where exchange rate is to be picked automatically into the billing document from OB08, we have defined a custom exchange rate type and maintained the exchange rates against it. we have assigned the same in the customer master, now surprisingly when we are creating a billing document exchange rate type M is getting triggered and values against M are getting copied into billing document.
Though exchange rate type shown in billing document header is our custom exchange rate type and not M and if i remove exchange rate values against M in OB08 the system throws an error.
I couldnot understand from where M is getting picked
also we have assigned the custom exchange rate type in the Accounting document
Kindly help us out, Thanks in advance