If you want to get super technical-here's the official meat cutter classification:
• Beef Loin, Bottom Sirloin Butt, Tri-Tip, Boneless, IMPS/NAMP 185C
This item is the portion of the bottom sirloin that remains after removal of the flap (185A) and the ball tip (185B). It is a triangular-shaped muscle and may have up to 1/4-inch fat covering.
• Beef Loin, Bottom Sirloin Butt, Tri-Tip, Boneless, Defatted, IMPS/NAMP 185D
This item is the same as # 185C except that practically all the surface fat and membranous tissue is removed.