i think it stands for manufacturer identification (MID)
it's like this - Memorex is not in the business of making DVD discs
so they have various factories (mainly in places like japan, taiwan, singapore,india...and some others) that make it for them and brand it as Memorex. Same for Sony, TDK, Fuji, Maxell and many others.
Some companies source their discs from "better" disc fabricators and other companies from "lesser quality" disc makers.
This is why you will see so much activity and interest in these forums relating to various discs - their quality, where they were made, who sells them and a ton of other information.
both CD and DVD discs should have MID codes and both vary in quality.
check out DvdInfoPro - a good tool to show info about your discs.