There’s pretty much two terms allowed by the rules:

Base code – pretty much the bare minimum code to get a blank screen up and initialize all your libraries. This could include project files, make files, an empty “main” method, or whatever boilerplate your platform requires.

Personal libraries – any game related code or utilities you have that does not implement a specific game.

In my uneducated opinion it is used in a very broad sense. Surely all the code that comes with Unity like the controllers, shaders etc. are sound, but i think many people here use libraries of their own making most extensively w/o declaring them specifically, because they are such general purpose routines, e.g. look at razoner who went through the pain explaining from which prior games he utilised the code for this LD, mainly because everyone was asking how he could accomplish so much in so little time.

Oh okay, thanks heaps guys. 😀