Sirloin tip is one name for a cut of beef that actually goes by several other names, and you may see it labeled as beef knuckle or round tip. That last term would be more accurate because ...

Both cuts are from an area at the back of the cow between the short loin and the round ... as "top sirloin." Bottom sirloin is chewier and leaner and can appear as tri-tip, flap steak or under ...