Here’s some further articles on Wikipedia that have helped me understand more about this:
Pulse Density Modulation. This is an SDM or DSD signal.
Pulse Width Modulation. This is the output from a Class D input stage (Modulator) to the power transistors.
PWM is a special case of PDM where the switching frequency is fixed and all the pulses corresponding to one sample are contiguous in the digital signal. For a 50% voltage with a resolution of 8-bits, a PWM waveform will turn on for 128 clock cycles and then off for the remaining 128 cycles. With PDM and the same clock rate the signal would alternate between on and off every other cycle. The average is 50% for both waveforms, but the PDM signal switches more often. For 100% or 0% level, they are the same.
Understanding ‘more’ is a relative term here. I have gone from understanding almost nothing to slightly more than nothing.