It's twin aisle 7 abreast, sitting in between the 737 and 787 in terms of pax capacity, with no cargo capacity, that will be lighter overall, and optimized for ranges under 4,000 or 4,500 miles depending on source. So a "sorta" replacement for the 757 and 767 with much better economics. And designed just as a people carrier. So expect to see it operated on high volume intra-Asia routes, and things like coast to coast or high volume intra-US and intra-EMEA routes. It'll also take some of the transatlantic market (e.g. anything served by a 757 today).
Operating planes designed for spanning the globe (777, 787, 350, etc) over shorter distances (<4,000 miles) just to get the extra seats becomes much less economical due to the weight of those planes and the wasted space on cargo capacity that can't be filled on the shorter routes.