Is the target reference piecewise defined anywhere?
 

Just came across a target reference labelled 'piecewise'.

Is this defined anywhere, and what does it do?

It's a deprecated function that was used for the skills setup in d20 but not used after that, which is why it wasn't included in the documentation. Matrices and arrays are better ways to accomplish the same thing if you're designing something new.

If you're learning from the d20 code, you are going to encounter a lot of things that are deprecated or that we now consider no longer best practice.