The Xbox One X is being marketed heavily as a 4K-focused machine, but it turns out that’s not the only way to get greater resolution than 1080p. The One X will eventually have native support for 1440p (2560 x 1440) monitors, as confirmed by Microsoft’s Kevin Gammill on Twitter. Gammill later clarified that 1440p support won't be ready immediately but is “coming soon.”
1440p is a popular resolution for high-end gaming monitors, particularly among players who like to prioritize framerate. While it’s unlikely that many developers will specifically target 1440p displays on the Xbox One X, the addition of system-wide 1440p output at least means that you’ll be able to make the most of the native resolution. The One X could theoretically also support variable framerate on FreeSync monitors, although nothing has been announced to that effect.
This is in contrast to Sony’s approach with the PlayStation 4 Pro, even though many games with PS4 Pro support are rendered at 1440p for the purposes of upscaling to 4K TVs. The PS4 Pro only supports 1080p or 4K output, and the latter only works with 4K screens. This means that if you play a 4K or 1440p game on a Pro hooked up to a 1440p monitor, it’ll just output 1080p and upscale, which doesn’t look great.
The number of people putting Xboxes on desks and plugging them into PC monitors probably isn’t high, but this is a thoughtful move from Microsoft that makes sense for the Xbox One X’s niche. While the One X is still very much positioned around 4K, and lower-resolution displays won’t often use the console’s power in the most efficient way, Microsoft is ensuring that they’ll still see some benefit with 1440p support alongside system-wide supersampling to 1080p.
Update, 2PM ET: Added Kevin Gammill's clarification that 1440p support is still in the works.