This directory might contain, for example:

• Lua files for usage alongside lualatex. Embedding Lua for lualatex to consume is quite powerful and neat, and we are gladly making use of it. However, embedding Lua verbatim into \directlua is a bit ugly and has annoying issues with escaping. Therefore, prefer to load external, proper Lua files using \dofile. These files might live in here, if required.
• Outsourced LaTeX source code that is better off modularized, like custom packages.