Do you mean for DVDs, BluRays and the like? It's the main source file from which all discs are pressed from. Same with Betacam masters. it is the source medium for transfers to other media.
That's why there are some members here that ask if there is any difference in quality between region 1 and region 3 DVDs--same with different region BluRays.
From what I know, Warner Bros. DVDs use the same master file/copy. That's why regardless of the region, the quality of the video is exactly the same across all regions because only one video file was used. One good example is Blade Runner: The Final Cut. And probably many other Warner Bros. DVDs and BluRays out there.
