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MODULATION GENERATOR SECTION

What Korg stubbornly refers to as a "Modulation Generator" is, in fact, a Low Frequency Oscillator (or LFO). It oscillates away, like the VCO, except in the sub-audio range (and it's not voltage controlled, but then very few LFO's are). What's somewhat misleading about the name Modulation Generator, is that it is only one of several modulation sources (the others being the EG, the Wheel, Portamento, noise and keyboard CV).
Aside from the name thing, however, the MS-10's MG is a simple, yet surprisingly versatile modulation source. It simultaneously outputs a variable triangle/ramp wave and a variable pulse (square) wave. The controls are a (very rare) waveform control knob and a frequency control knob.
Internally, the triangle/ramp output is pre-patched to the modulate the VCO frequency and/or the VCF cutoff frequency. Via the "Wave Form" knob, it can be adjusted from descending ramp wave, to triangle wave, to ascending ramp wave.

Modulation of the VCO with descending ramp wave ("Wave Form" knob all the way counter clockwise).
Modulation of the VCO with ascending ramp wave ("Wave Form" all the way clockwise).
Modulation of the VCF with slow triangle wave ("Wave Form" knob centered).
Modulation of the VCF with variable ramp/triangle wave ("Wave Form" knob being swept fully clockwise). Although sweeping the waveform must be done manually, it is a feature found on very few synths (hence a slightly larger sample to fully demonstrate the effect).

The triangle/ramp wave output is also available via the patch panel and outputs a voltage of from -2.5 V to +2.5 V. As such, if it is used to modulate the VCA (via INITIAL GAIN) only half the cycle of the waveform is recognized. It's still a moderately useful effect (and is described more fully in the VCA section). Aside from that, since the triangle/ramp output is already pre-patched internally to the VCF and the VCO, the only additional destination offered by its inclusion on the patch panel is Pulse Width Modulation (as described on the oscillator page). {Of course, if one also has another MS-10, or a 20 or 50, it can be used as an additional modulation source there as well.}
The pulse wave output of the MG is only available via the patch panel. It produces a voltage of 0V or 5V and can be used to modulate any of the following:

VCA, although the affect is more jagged than with the triangle modulation.
VCF cutoff frequency modulation. (MG square wave output patched to VCF CUTOFF FREQUENCY input; "EXT" knob in Cutoff Frequency Modulation section of the panel swept to vary level).
VCO frequency modulation (MG square wave output patched to VCO FREQ input; "EXT" knob in the Frequency Modulation section of the panel swept to vary level).