Sirloin tip is the traditional name in United States. Elsewhere, butchers sell the same piece — or a close cousin — under a different name.
Both lean round subprimals. Bottom round (silverside) is the harder outer-thigh group; sirloin tip (knuckle) is the front-thigh quadriceps and is naturally tenderer.
Same anatomical muscle group — the knuckle at the front of the round. Bola de lomo is the Argentine retail cut; sirloin tip / knuckle is the US–UK name. Same cut, different tradition.
Both round-area muscles, but anatomically distinct — sirloin tip is the front-knuckle quadriceps, poire is a smaller pear-shaped muscle on the inside thigh. Poire is the French-butchery boucher's secret, sirloin tip is the broader US/Anglo cut.
Sister cut on the round — sirloin tip is the front knuckle (quadriceps), topside is the inner thigh (adductor). Both lean roasting cuts, sirloin tip is the slightly tenderer of the two.