Jeff Epler is a modder. While he admitted that modifying the game resulted in less flashy changes in comparison to level editing, he preferred designing modifications.
Epler is notable for making all of his modifications as .PAT files as opposed to the standard .QC files. This requires an executable known as PATCH.exe, which reads the .PAT file to compare differences between older and newer versions of the .QC files. The .PAT file can be therefore used as a changelog, complete with a relatively simple manual transfer, which may be needed since PATCH tends to get errors and reject patches. "+" states the line is being added, while "-" states the line is being removed.
The benefits to .PAT over .QC, and reasoning for Epler to use it, is that the files can be smaller since only the changed lines need to be documented. It also provides a clearer difference between pieces of code, which in turn allows for easier extensibility on top of other modifications.