For instance I want to reference the build volume maximum Z-axis value in my start-up script (to set the Z-reference).
Yes, I can type the value into the script but I presume it is also one of the variables I could use via the [ ] option... but as it is not one of the very few variables shown in this sparse post you keep referring everyone to and there is no manual I'll never know will I?
Another for instance - a script fragment is shown to set the extruder temperature using the layer 1 value... what if I want to use the value for a different layer?
I paid for this product - I expect there to be a comprehensive technical manual or at least a manual! I don't want to dredge through the user forum for every little bit of information.
That link is already comprehensive. If the specific thing you are looking for isn't shown at that link, then it doesn't exist.
For example, the only temperature variables that exist are for the first layer temperature. Those are all documented at that link. There is no such thing as a variable for the 5th layer temperature like that.
So if there's extra options or variables that you want, you are welcome to request them, but that link I posted already contains the comprehensive list of what currently exists in the software.