ZO_ Lua objects?

[Tingle0x539]

I've noticed the use in lua of called things like ZO_WorldMap. Nowhere in the API wiki or google could I find any reference to these, can someone tell me where they are? I take it they are the XML Elements from the games UI? Where can one get insight to these?

Also, using things like $(parent)somestring for XML names like used in the MyFirstAddon example, where does this come from? I could not find any reference in XML, Lua or a generale google search that shows such a method of merging strings.

I have also seen files like this: https://docs.google.com/file/d/0B36M...MkU/edit?pli=1
Where are they mentioned or released?

Thanks,