I disagree with a lot of what Grayeaglem wrote.
The cap steak is also called the culotte steak -- you may be more familiar with the term and be better able to find recipes. It does come from the top sirloin butt. Unsurprisingly, it's a "cap." A unique muscle on the larger butt, separated by a layer of fat and membrane.
It cooks exactly like a "tri-tip." You can marinate it, but it's not really necessary. A decent piece doesn't require a jaccard. Broil or roast, do not braise. Cook to medium rare.
The keys to tenderness are the quality of the animal and slicing. A tender side of beef will produce a tender culotte, and a tough beef a tough culotte. Go for "Choice" or better. It's a cut with a very distinct grain, and if you leave long fibers they will be hard to chew. So, slice thin and across the grain.