In Mach3 Turn, under General Logic Config halfway down the right hand side of the tab, there is a box labeled "Turn Manual Spindle Incr." On my system the box contains 50.
Does anyone have any idea what this is for?

Neither it, nor the box under it labeled "Spindle 0V increment" with 10 in the box, appear anywhere in the Mach3Turn_1.84.pdf manual (or in the Mill manual).
But then the pics of various tabs in the manuals do seem to date back to Mach2 in some cases, so ...

Cheers
Roger