Hightower, the tri-tip comes off the bottom of the sirloin. Way back when I was cutting meat, this was usually cut into cubes for beef stew or ground into hamburger as there is only two tri-tip strips per beef. Markets now get boxed beef and so the tri-tip is now more common. Back in my day, we had to "break down" the front and rear quarters of beef.
Here in the lower 48 the tri-tip is marketed as tri-tip or as a triangle roast.