As you know the stepper motor has two coils that must be energized singularly and together in sequence and in forward and reverse directions If this is not apparent then you need to revisit the section on the basic stepper motor theory.
To do this the driver needs two bridge circuits such as Fig 33.
The FWD and REV signals are supplied from the controller which is in actual fact a computer.
The computer is programmed to provide the two sets of FWD and REV signals one set for each driver for each coil.
The computer program determines the order that each set of FWD and REV signals are applied and looks after the problems of acceleration and deceleration.
The user of the software instructs the computer on how far the stepper motor is to turn i.e. how many revolutions and in which direction, and the computer takes care of the rest.