I agree with you the numpad keys could be better documented. Here are the most common uses:
- text input for project names: the black keys are mapped like a phone keyboard with [abc] [def] [ghi]. You can see a visual next to the text input windows that shows the mapping to characters
- sequencer: left column are the enable/disable buttons for the four pulses.
- audio effects and assignable modulators: top row of three black buttons are extra parameters in some audio effects/modulators. What parameter it controls is shown on screen. For example here the buttons control trig, shape and range.
- quad engine: middle column can be used to select what sound plays (kick/snare/sample etc) for each of the four pulses
- FM engine: buttons set the modulation matrix / fm algorithm. See FM - PolyPulse manual