Beef sirloin is one of the two major subprimals of the beef loin primal cut, which runs from the 13th rib all the way back to the end of the hip bone.
Of those two subprimals, the sirloin is the one further back toward the rear leg, where the muscles get more exercise, making them tougher (the front part of the loin is called the short loin). The sirloin is separated from the short loin at the front tip of the hip bone by a straight cut through the seventh lumbar vertebra.
The sirloin is almost always broken down into two boneless wholesale cuts; the top sirloin butt and bottom sirloin butt. This is done by cutting along the natural seam between the gluteus medius, the primary muscle of the top sirloin, and the knuckle, a group of three muscles (the rectus femoris, vastus lateralis, and vastus medialis) sometimes called the sirloin tip.
The top sirloin butt (also known as top butt or sirloin butt) is usually sliced into individual steaks, and it can be trimmed to varying degrees first. For instance, the top butt features a triangular muscle called the biceps femoris, or sirloin cap, which is usually pulled off and made into steaks.
The main reason for removing the cap is that the muscle fibers run in a different direction than the rest of the top butt. Since these are less tender steaks, slicing them against the grain helps make them more chewable. Separating the two muscles allows each one to be sliced against its respective grain.
Remember that top sirloin, top butt, and sirloin butt all mean the same thing. Top sirloin steaks are generally suitable for high-heat grilling, although you definitely want to be careful not to overcook them since they can be tough and dry. Marinating sirloin steaks is a great idea because it will add flavor and moisture, but it will not help tenderize.
Bottom Sirloin, Tri-Tip, Sirloin Tip, and Flap
Just as muscles toward the rear of the animal are tougher than ones from the middle, muscles from lower on the animal are tougher than ones from higher up. With bottom sirloin, the muscles are getting tougher. Cuts from the bottom sirloin tend to be used for roasts, and it provides a lot of ground beef and stew meat, too.
Probably the most popular bottom sirloin roast is the tri-tip, which is made from a coarse-fibered triangular muscle called the tensor fasciae latae. Tri-tip is fairly lean, although it does have a layer of fat on the outside which is sometimes trimmed away, but which can be desirable if you're cooking it slowly. Tri-tip can be grilled over indirect heat or smoked/oven roasted at a low temperature (i.e. 225 F). The key is not to overcook it since it will dry out and become tough. Some cooks like to season tri-tip, and since the meat is so lean, it does benefit from a little extra flavor. It also benefits from marinating. Whatever you do, be sure to slice it against the grain when you serve it to make sure it's chewable.
Sirloin Tip or Round
The sirloin tip (or knuckle) is another roast from the bottom sirloin, and it happens to be situated precisely at the spot where the loin is separated from the round. If a carcass is butchered according to specifications, about 3/4 of the knuckle will end up in the round section, and the remaining 1/4 of it in the loin. What often happens instead is that the whole knuckle is removed from the carcass and sold as sirloin tip.
Admittedly, the divisions between primal cuts are sometimes arbitrary, and the knuckle is the knuckle, regardless of how it's divided or what it's called. Still, calling something a sirloin tip when it comes from the round is misleading; it's inaccurate, and the price per pound for something called "sirloin" is probably higher than something called "round."
Finally, the sirloin flap is a thin muscle called the obliquus abdominis interni, located beside the tri-tip, where the loin curves down toward the belly or flank. In fact, the sirloin flap is very similar to flank steak—coarse-grained muscles with lots of marbling and deep beef flavor. Marinated, cooked over high heat to medium rare and sliced against the grain, sirloin flap is a delicious piece of meat.
Tenderloin or Butt Tender
Finally, the sirloin sometimes contains what we call the butt tender, or rear portion of the tenderloin, which is the most tender muscle on the beef carcass. Since it's at the wide end and has a fairly uniform thickness, the butt tender is easy to make into steaks. It can also be trimmed and sold as a whole roast. However, the tenderloin is often removed from the loin and either sold whole or as tenderloin steaks (i.e. filet mignon) or roasts (Chateaubriand).