Long time user of Mach3 but new to Mach4.  I keep seeing references to the Mach4 "Customization Manual" in the Operators and Scripting manuals.  Where can I find this?

Thank you Steve.  More specifically, I am looking for the manual referenced here in the scripting manual:

3.2 Screen Scripts
The screen contains scripts than run on load and unload and a plc script. Certain screen elements (such as buttons, panels, DROs, and tabs) can also execute user defined scripts. These scripts call all be set in the screen editor (see the Mach4 customization manual for more information of customizing the screen).

The Scripting manual says to see the Customization Manual.  I don't see a Customization manual in the link you provided? 

I am fairly sure that the manuals on that page are all that there is.

It would be nice to have a general overview of how the internals work and
how to access internal registers. Someone did a list of registers once, but
it is probably out of date now.

I don't think a Customization Manual exist. This link should help. https://www.youtube.com/watch?v=TH944Qq86kc

It will not help much with lua. This link will though. http://www.lua.org/manual/5.2/

Between it, the existing scripts (in the default screens wizards and modules), the forum topics in the Mach4 boards and the Mach4CoreAPI.chm in the docs folder they should give you a good toe hold. Customizing Mach4 is the easy bit. Learning lua so you can do advanced customization is what takes time. 

Thanks for the links I will check them out.  I have some code experience so I am more concerned with the general workflow of customizing Mach4.  I was thinking there was another manual that I couldn't find.