A programming flag used to disable one or more functions of a media player, such as Stop, Menu, and Chapter Skip. On a compliant player, it forces the viewer to wait as certain material plays before the main menu becomes available. Originally, this was used to require the viewer to see the copyright warning, but it has since been regularly abused to make advertising and promotional material unskippable. This is one of those things that sounds great to home video producers but the logic of the practice falls down when you realize that the people who will be affected by this stuff obviously own a legit DVD, and the people who pirate the video won't get (or will be able to skip) the warning or ads. In other words, good behavior is being punished with annoying ads, and bad behavior is being rewarded by the lack of them.
The formation of the User Operation Prohibit Flag was arguably set in motion during the V/H/S generation, where viewers could skip the trailers running before the feature presentation by fast-forwarding through them (LaserDiscs almost never had previews). The leap to DVD and Blu-ray, which offer scripted seamless branching (as well as the rise in digital distribution and piracy) forced production companies to institute DRM that holds the viewer's attention while it shows promos for their latest works. That is, latest at the time of the disc's pressing. This can range from a single trailer to several minutes worth of commercials and advertisements for unrelated media. If the disc is old enough, the "new" media in question may even already be out of print, even on the company's "official" shop, making the whole "trailer" sequence completely pointless. (With the rise of Blu-Ray, the BD-Live function can avert this depending on the disc, as the function enables the disc to download fresh trailers over the Internet.)
As one might imagine, this playback behavior fills many people with a vast, seething hatred. People who own DVD editing software like HandBrake may even be tempted to copy it purely to get a version without all that stuff. If you're lucky, your player might have a way around this, perhaps by pressing a certain set of remote control buttons. On some players, just hitting "menu" works sometimes. Some players will also hiccup if you try speeding up the playback past the tolerance point; dumping you at the menu. Of course, just as many will hiccup and go back to the beginning.
Unsurprisingly, the User Operation Prohibit Flag is slowly creeping its way into online distribution services as well. In 2006, Amazon's Unbox service was released with a service agreement that barred the customer from turning off their software, auto-updates without their consent and puts commercials and trailers on the user's computer without their permission.
DVD player software for personal computers, such as VLC Media Player, can often skip these even if flagged as unskippable.