What is tpzi waveform?

They give different type of waveform as output. Some inverter UPS manufacturers name their products as digital inverters, modified-sine wave, Trapezoidal waveform, stepped sine wave, quasi sine wave etc. Sine wave inverter produces an output as close to the grid supply as possible (a sinusoidal waveform). My power inverter produces trapezoidal (TPZi) waveforms instead of sine waves. I have written about the trapezoidal waveform before, but I have never.