BI Architect expressions

I work a lot with IRIS BI, but some features are still a mystery to me. There is a great feature here that allows you to create expressions in Architect.

  

I'm already very familiar with it, but I still don't quite understand what environment it uses.

It can execute system classes like $SYSTEM.SQL,  $PIECE(), %cube, %source(), it can do string concatenation, call methods from another classes.

I have two questions actually.
What environment it uses to execute this? Is it ObjectScript, SQL, MDX or maybe even its own? I need to know this to understand the mechanics behind it, how the Architect work

Second, what are the FULL list of what it can do and what it can not. 

I looked through the "wonderful" documents on this and found several pieces of information scattered over 3-5 pages.