Fuzz was one of the first “transistorized” guitar effects. Transistors amplify the signal and when they clip, distortion is generated. When the clipping is hard instead of gradual, the distortion is harsh and buzzy, named “fuzz”. The tone of fuzz depends greatly on the type of transistor used: germanium, silicon etc. (available as Clip Types in the Drive block).
I have a problem with this description as applied to the Pi. Everything in it is true - when you're talking about an actual fuzz circuit. But the Big Muff Pi, isn't a fuzz design. Yeah, it has a big, woolly distorted sound, but the circuit isn't anything like a Fuzz at all.
First off, the Pi uses diodes for clipping. So the, "and when they clip, distortion is generated", is irrelevant. The Pi does use cascading gain stages, but this is just to pile clipping on top of clipping.
As far as I know, the Pi has always used silicon BJT transistors. Common as dirt and no big deal, and the exact model isn't considered to be a big contributor to the tone or nature of the clipping. In the 70's I did own an opamp version of the Pi (which used a chip to provide the amplification to drive the clipping diodes), so even EHX didn't considered the transistors to be a vital part of the sound. The opamp version, however, really was nowhere near as good as the transistor version.
So the "The tone of fuzz depends greatly on the type of transistor used: germanium, silicon etc. ", doesn't really apply here. Big Muffs never used germanium transistors.
Finally, "(available as Clip Types in the Drive block)", is probably not correct either. I'm not sure how Cliff translates the "clip type" parameter with the FF design drive blocks, but I'm fairly certain this is going to refer to the clipping diodes for the Pi model. The actual Big Muff uses silicon diodes.
I'll admit that calling a BMP a "fuzz" is one of my personal hot buttons. To me, it doesn't sound anything like a real fuzz, and I don't see why you'd lump it in with them. Under the hood, though, it's clear that fuzz and BMP have NOTHING in common. Nothing. Nothing. In truth, the BMP is a unique design using discrete transistors with diodes in a feedback loop, and then running two of those clipping circuits back-to-back. So I think it deserves to go in a category all its own.
All of that is just a rant, but when you take that misconception and apply it to a description of the circuit to explain how it's modeled in the Axe, what you end up with is erroneous.