I assume you are talking about a Bernina that has the attached drop down dual feed foot? Anyway, if you want to use the dual feed foot to help feed the fabric evenly, you do want to get the D8 foot. I think it really does help with feeding the fabric though. I got the "D" foot set when it was on sale one time...the bummer was that I repeated a couple of the "C" feet I already had. But I use the "D' mostly.