It is explained on page 4 of the documentation. Vm is the power
supply pin for the chip and the steppers. It takes +7.2 to +16.5
vdc. Vd is used for a clamping diode between pins 1 and 16. Any
time you turn power on then off with a coil in a circuit (the
motors), you will get back voltage returned from the coil. This
voltage can rise pretty high and spike components in that
circuit if not dealt with. If you look at figure 12 on page 9
you can see the voltage will be returned to the supply + side.