Not Encyclopedic
This is a product description from a catalog rather than an electrical or acoustic explanation of the effects:
Filter and synthesizer effects:' Pedals such as the MF-105 moogerfooger MURF provide multiple filters and envelope control knobs to control modulation. The MF-107 FreqBox uses the input signal to modulate an internal VCO oscillator.

Also, how is a filter effect a modulation effect?