It's basically DTS teaming up with a brand name (IMAX) that is associated with spectacular movie playback, in order to better compete in the market place. The idea is to mimic aspects of IMAX that people really like: detailed & bright picture with dynamic & immersive audio. The video part will be 4K resolution with some sort of HDR.
Most movie soundtracks have their dynamic range slightly reduced when re-mastered for home video. They have to play back on TV speakers after all. IMAX Enhanced will be an opportunity to maintain the maximum dynamic range possible. Also, IMAX theatres have a Centre Height speaker at the top of the screen, for a 7.1.5 speaker configuration. Most DTS:X tracks so far have been 7.1.4, so they will likely add an extra channel (Centre Height). IF your set-up doesn't have a Centre Height speaker, sound from that channel will simply split to the Left & Right Height speaker (same as if you have no Centre speaker).
So far, two studios (Sony & Paramount) have signed on to create IMAX Enhanced mixes for some of their movies released on 4K UHD. IMAX themselves plan to release some of their documentaries in this format as well. And some manufacturers (Sony, Denon, Marantz) will be making receivers and pre-pros with IMAX Enhanced playback mode. The current Denon & Marantz flagship models will be getting a software update in October that will give them the IMAX Enhanced mode.
As with all new formats, it will be completely backwards compatible with DTS:X receivers that don't have the IMAX Enhanced mode as well as regular DTS receivers. Heck, they'll even play back on the two speakers on your TV.
All movies come letter boxed, but even in such movies some scenes come for full 16:9 screen. You can see this in all Nolan movies, it switches between letterbox and full screen in movies. Other than that I have no clue why another such standard and certification. May be a ploy to make us buy more stuff or upgrade !