Oh yes, I see the link to BSS website in post #4. The ligature is described as having "3 distinct radiuses" and no reference to reed planes or trapezoids.
Yes, I like to add my own reviews and experiences in addition to the company's own description. A trapezoid has 1 side that is longer than the other, and in the visual, you can see the top is wider than the bottom. That area above the reed is what I call the reed plane, as it directly interacts with the reed. A literal 2D plane doesn't exist in our world since everything has depth, so its the flat area of the ligature above the reed, which is also curved to fit the reed (see first post).
Here's a rough image I threw together to explain it. The full video will go over the shape in more detail. I consider the plane starting with the distinct bends on top and near the bottom. They are a result of the top of the ligature having a different shape than the bottom. You can also see the reed plate that has an eye-shape cutout.
When on the mouthpiece, the trapezoidal reed plane keeps the sides of the reed open, whereas a Selmer 2-screw with a rectangular plane will be more squished on the top.