Picanha, coulotte and top sirloin cap are the same cut wearing three names. The spec underneath all of them is IMPS 184D, Top Sirloin Butt, Cap, Boneless: the triangular terminal portion of the biceps femoris muscle that sits as a cap on the top sirloin butt, carrying its fat cover.
The three names
Picanha is the Brazilian name, and in the churrasco tradition the fat cap is the point: the cut is cooked with the cap on and served in slices through it. Coulotte is the common US foodservice name, and coulotte product is frequently trimmed of most or all of that fat cap. Top sirloin cap is the plain descriptive name. Same muscle in every case. The practical difference between a picanha spec and a coulotte spec is usually what happened to the fat.
Why the distinction costs money
Because the names imply different trim levels, a quote is only comparable when the trim spec travels with it. Cap-on product carries more of its total weight in fat than a peeled one, so a lower price per pound on cap-on product is not automatically the better buy. The IMPS number plus explicit trim language settles it: 184D with the fat cover stated is unambiguous where a menu word is not.
Where it sits in the market
The cap comes off the top sirloin butt, so its supply rides the same end meat complex as the top butt itself, and demand for it has grown with the spread of churrasco style dining in North America. It stays a small cut relative to the primal it rides on: each carcass yields two of them, which is part of why a popular name can move its price faster than the primal underneath moves.