PWM Schemes describe the way the gates of the three half bridges are being controlled.
Most common PWM Schemes are so called trapezoidal High PWM Low on and trapezoidal High on Low PWM but there are also other kinds of trapezoidal PWM schemes.
Trapezoidal PWM Schemes
Here some pictures of the different possible trapezoidal PWM schemes:
H PWM L ON
H ON L PWM
PWM ON PWM
Sinusoidal PWM Scheme