Scanner (galvanometer) position detector

I have find CamTech 6215 scanner datasheet, here is it: https://physics.ucsd.edu/neurophysics/Manuals/Cambridge/Cambridge%20Technology%20Model%206200%20Optical%20Scanner.pdf
http://www.edmundoptics.com/lasers/laser-mechanics/dual-axis-galvanometer-optical-scanners/86809/
If I get voltage +/-3V on "+motor" and "-motor", I able to see the movement of mirror. It is good.
But what these abbreveatures are meaning: https://monosnap.com/file/S9sA9YMyPjp5q2NK2Yglc7kOKeIp6m ?
How can I detect current position of mirror?