There are 3 modulation source on the Pro-1 on the left hand side of the unit, labelled FIL ENV( Filter Envelope), OSC B and LFO. These incorporate mix dials and allow you to route different functions of the synth to the sources.

Each dial determines the amount of modulation caused by the mod-wheel or direct route as determined by the position of each switch. If DIRECT is chosen, the modulation will be applied directly to the destination. If WHEEL is selected, the modulation will be sent to the mod-wheel on your controller which then determines the amount of modulation applied to your destination. The destination in then chosen by the switches in you 'TO' column.