The default output from an operator is a sine wave, but you can use the HARM parameter to add upper partials to some of the operators’ sine waves to create more harmonically rich waveforms. The HARM parameter is bipolar. Negative parameter values change the harmonics of operator C, while positive parameter values change the harmonics of operators A and B1.

When changing harmonics, intermediate values interpolates between the current and the next harmonic. This interpolation works much like in wavetable synthesis, sweeping between the harmonics, smoothly
transitioning from one timbre to another. The harmonic series for the operators looks like this:

The Digitone uses a form of additive synthesis to create the harmonic series for the HARM parameter. Additive synthesis is one of the oldest forms of electronic sound generation. It is a quite simple form of synthesis but still very powerful. The basic principle is to add multiple sine waves together to form complex timbres. Each sine wave is called a partial. Typically, each part is one octave above the previous. You retain the frequency of the tone by keeping the partials spread across octaves. By attenuating each partial, the timbre changes its harmonic content, resulting in different waveforms.

The first partial is always kept at full volume, which keeps the base octave intact.

Adding every partial in series will replicate a sawtooth. Note how the volume decreases for each partial, creating a natural falloff.

Adding every odd partial in series will approximate a square wave.

The additive method can be used to make many different timbres. This combination of partials, for example, sounds close to a bell tone.

Read more about the Digitone FM synthesis here:
Digitone FM synthesis overview.