"how a piece of Python code is distinguished for a division to be made in the interpreter (either C program or Java classes as the author states)"

Below are some pointers that might answer your question :

Usability :

1. CPython - Most commonly packaged when you download python(on windows) or install linux(pre-installed). So can be used for almost any python project with exception mentioned in point 2.
2. JYthon - You have to specifically download the jar for this from JYthon official website. So if your python project is going to require access from Java libraries and/or frameworks, JYthon is ideal interpreter.

So to answer in short, its not your piece of code that will decide which interpreter but you the user based on your project architecture(whether it has java library/framework dependencies) decides whether to use CPython or Jython.

Note : For a project both CPython and Jython can be used together, but remember only one interpreter can use the run-time environment at a time.