Pin-Out Optical Encoder Question

Hi -

I am looking at this link: **broken link removed**

Under the Pin-Out section, it says:
1 Ground
2 Index
3 A channel
4 +5VDC power
5 B channel

What is #2, Index and what should I connect that to?

Thanks.

https://usdigital.com/assets/general/74_e2_datasheet_4.pdf
That says that the index provides a single pulse per revolution.

Somewhere else it said that the index is optional, so I don't know if it is fitted. Just don't connect anything to that pin if you don't want one pulse per revolution.

The index pulse can apparently be used if you want to detemine a "zero" position of the encoder.