diff
TextPackage: diffutils
The differences between two files, line by line. diff -u produces the format git and patch read, and it is what goes into a report to show what changed in a configuration.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
diff -u- Unified format: a few lines of context, removals prefixed
-and additions+. It is the form everyone reads, and the form a patch takes.