Linear glass scale pin 9

Just completed building and testing electronics for the Yuri Touch Dro system.
Originally thought about using rotary encoders with 600 ppm operated by wire but when thinking more about it, resolution is too low.
Looked at glass scales with ttl output and wondering what is the pin 9 I see in many instances.
Can see 0v, 5v, shield connection, A and B outputs but the pin 9 has me stumped as to what it is used for.

https://ncmaster.com.au/ncmaster/ind...product_id=444

My guess would be Reference = Z = Index

What he said. Reference (index) signal.

OK...thanks for that.

Another query....
Linear scale supplier quotes output as "TTL square /wave EIA-442" so I imagine if I only need the TTL signal, just use A and B out?