You have control of input gain via 'load' pot instead of 'fuzz' seen in the most FFs, what allows to have some flexibility in available amount of distortion and keep clarity in a whole range. No clean setting though - did I mention it is brutal? 'Load' knob is a handy tool to use with another pedals before FFAS, coz when turned back it 'unloads' preceding device so you can use your wah, booster or other pedals with no worries.
Depth pot allows to reduce both amount of bass and overall heaviness, which turns the tone from really bulky, beefy fuzz to somewhat lighter and cutting through fuzz with pronounced midrange.
Level knob is simply a volume control with lot of gain available so you can push your amp savagely. Turning it up also provides a little more aggressive and bright tone compared to 'unity gain' setting.
level - output volume level
depth - turned CCW reduces low end
load - turned CCW reduces gain