This nomenclature was often used in licensed Quake codebases to indicate places where a vendor would put their own Mission Pack-specific alterations into the code. I guess they just 'up-merged' the modified code to the main id source when they were done, or perhaps the changes were done by id on behalf of them?

I believe there were similar bits in the Quake 1 source, e.g. for Scourge of Armagon (Mission Pack No. 1) you would see things like '// HIPNOTIC'.