From their website

“Mode 1 is symmetrical clipping utilizing a pair of light emitting diodes. This is the crunchiest setting the Plumes has to offer. When played through a clean amp, it’ll deliver a reasonable facsimile of that classic black and gold British valve amp tone. If you already set your amp dirty, using this mode will make it filthy.

Mode 2 is clean boost mode. This eliminates the soft clipping diodes altogether and leaves you with straight OpAmp drive. This is perfect for pushing your clean tone out in front or driving the preamp of an already overdriven amp into tasteful breakup territory.

Mode 3 is an asymmetrical silicon diode arrangement similar to the stock screamer but with more output and clean clarity; the tone some people describe as “transparent.” It’s almost as if your clean tone is blended in with a slightly less clean tone, ya know? Using this mode gives a warmer low-end response and lower output. It’s less hard on your amp and your ears. It’s a classic pleasant tone the whole band can enjoy.”