In fat transfer, the cannula is the hollow tube that is used to deliver the fat to the tissues. The tip is usually blunt for safest application. The size depends on the body area. We use larger cannulas for fat transfer to butt or breasts, smaller more precise cannulas for facial fat transfer.
I've been doing fat grafting since the 1980's. I've tried every permutation of injection cannulas and I don't think the tip design makes a difference. Diameter has a modest effect