[SS10] Named Expression documentation?

Named expressions sound like a great way to access a lot of information about the file, system, elements...But I have no idea how to write one.

Is there any kind of thorough documentation anywhere?

My main interest at this moment is for pen table text substitutions, but I suspect they would be far more useful than that. I just have NO idea where to start learning. My searches turned up some interesting tidbits, but it seems as if everyone already knows how to write them, they just need fine tuning. I have no idea where to start.