Where is a document that describes the Settings screen (f6)?

My setup appears to have position indicators for each drive motor, so the absolute location is shown in the Diagnostic screen.
When I press the 'zero' button in the settings screen for the X axis, I expect to see the DRO get set to 0.000, but is is usually a different value. Lately it has been 0.029 in.

I am not having any success with the 'intuitively obvious'.

So, is there a manual which would explain the Settings screen?

JA