Pulse+ Pulse- with stepper motors
It may just be the anode and cathode polarity indicators for an optically isolated drive signals.  Here is an example product with similar terminology from Rhinomc RMCS-225X:

http://www.active-robots.com/dc-servo-motor-10rpm-uart-12c-ppm-drive



If you open the datasheet you will see the internal diodes. They leave all of them independently floating in order to allow the user to drive them as needed. (Common anode, common cathode, differential).