Before posting here, did you check with your internal team as to why system is not fetching the right document type? From your post, I feel, you have not checked. Since you have indicated that you are new to SD, first and foremost fact you need to consider is any issue you face, you have to first discuss internally. If your internal team are unable to provide a solution, then you have to search SCN. Finally, check whether any user exit is in place to trigger a customized billing type
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