The DM100 has four user-controllable parameters - Mode, Distortion, Tone, and Level.
Mode: Use this switch to toggle among three different distortion simulations - DS1, which is a hard distortion with subtle nuances; D+, which provides a retro-style soft distortion; RAT, which is a smooth distortion with sustain and plenty of cut-through power.
Dist: Use this knob to adjust the amount of distortion effect.
Tone: Use this knob to adjust the overall shape of your sound.
Level: This knob adjusts the overall output volume.
As always, experiment with the different settings until the you get the results that sound best to you.