On the attached snip, what is the purpose of the RPM line circled in red? At the bottom where the two zeros are I can see my spindle RPM but the red circled line always shows 0

Is that not the line that shows rpm from an encoder input?

I think my setup uses that line and it varies with load on the spindle etc.

Yes, it requires an input on the index input to provide the actual rpm to Mach3.

Thank you both