In its simplest form, a quantizer is a module that takes in a control voltage, and outputs the same voltage rounded to the nearest note. In the usual V/Oct tuning system, and assuming we use just tuning, that means rounding to the nearest 1/12th of a Volt.
An obvious extension to such a basic quantizer is to have adjustable scales, where some of the 12 notes are disabled and hence will never be output. For example a quantizer could be set to a major scale to make sure the VCOs will always play only notes from that scale.
In my Quantizer I chose to have completely flexile scales, defined by 12 illuminated push buttons that are toggled on and off by pressing them. This allows any custom scale to be programmed, or for example only a subset of a scale to get more control over the resulting music. I often find enabling only 3 or 4 notes gives more useful results than a traditional 7-tone scale.
Rather than the traditional piano keyboard layout, I use a circular layout of the scale buttons that is comparable to the concept of bracelets. This layout is to me more intuitive compared to the piano keyboard - for example transposing the scale is just rotating the notes on the ring. As shown below, on the front panel layout I still made shaded regions to indicated the black keys on the piano keyboard for reference.