I use this module as part of my complex oscillator setup. This is basis for my complex OCS. FMOS run a sin wave usually one or two octaves below the VCO. The VCO is running a triangle wave into the SIGNALAMP. Also the FMOS out goes into the MODDEPTH of the VCO. This saves on VCAs.
We all know the original VCO is not the greatest, but the combination of the digital FMOS combined with a very precise way of adding CV, makes the original VCO very stable indeed. In other words a signal from the quantizer goes into slot A of the 2CVADDER. The outs from Slot A go to the inputs on both oscillators.
This also makes sure that both oscillators play the precisely the same pitches and takes average tuning out of the equation. The problem with frequency modulation is that the average intonation changes the wider the interval between the oscillators. The dadgum this won't play in tune, especially with complex sequencing that I do or with generative music I make. So, to get both oscillator moving at the same rate means the average intonation doesn't change because of the precision adder.
For instance WAVETABLES already has this effect. I don't know if is has a precision adder behind the hood but notice whenever you change the pitch and use the second oscillator, it automatically changes and stays in tune relative to the interval between the two oscillators. It makes FM far more usable in quantized music.
Also changing keys/offsets is easy. Just 2ATT set to 5v and you can change keys all day and both oscillators with play the same pitches all the time no matter the range. Or us a second quantizer or sequencer to control the pitch changes, and you can do proper key changes.