What the shipped code is, and who wrote it. The area divides by authorship rather than by language: some of what ships is written here, and some is vendored from elsewhere and described only as far as this package's build of it differs from a stock one.
Little of what ships was typed where it is read. A source file here is as likely generated from an annotation, derived from a sibling, or copied from upstream as it is hand-written, so the first thing a leaf says about a file is which of those it is — and a correction goes to the producer, never to the file in front of you.