https://www.theverge.com/circuitbreaker/2017/11/2/16597100/xbox-one-x-1440p-support-confirmed

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. Unexpectedly, the One X will also natively support 1440p (2560 x 1440) monitors, as confirmed by Microsoft’s Kevin Gammill on Twitter.

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.