In FM synthesis the oscillators are called operators. Unlike the analog oscillator, the operator also contains an envelope and specific input and outputs, making it a sort of macro oscillator. FM combines two or more operators to generate a more harmonically rich output. An operator that is used to modulate another operator is called a modulator. The operator that generates, or carries, the resulting tone is called a carrier.
Feedback is used to increase the sharpness of a sound. Feedback is only applied to modulators. The output of the operator is fed back into itself, resulting in added harmonics. In Digitone, an operator with feedback with a parameter value around 35 results in a sawtooth waveform. If pushed to higher values, the feedback soon results in noise.

In some instances, an operator can be both carrier and modulator. This means that it outputs its result as a tone while also using it to modulate another operator. We have divided the operators into three groups: C, A, and B (B1 and B2) to lessen the complexity and make the Digitone easier to use. Since group B consist of two operators, the parameter controls for B are macro mapped to both operators.

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