Ants covered it pretty well. The Seat and Feed Die is for use with the Bullet Feed Kit. The bottom edge of the Seater die is beveled so that the Feed fingers will operate properly.
I set up two of these on my Load-Master two years ago--and gave up on them--for the reasons ants cites. Even if you can get them operating 'ok'--and I eventually did, with a 10mm setup--the time spent filling tubes, etc., cancels any benefit. And, since they really don't work well, they are only an exercise in frustration.
IOW, don't bother.
Jim H.