Sony's Concept is an optional program for Xperia X owners, which switches out the custom Xperia software for a nearly-stock Android ROM. Android 7.1.2 was released for Nexus and Pixel phones only two weeks ago, and it looks like Sony will be the first OEM besides Google to push the update.
If you need a refresher, 7.1.2 improves Bluetooth connectivity, adds Battery usage alerts, and includes many bug fixes. Sony notes that the fingerprint swipe gestures are still not available for its devices, likely due to the same firmware situation that delayed the feature for the Nexus 5X and 6P.
The update should arrive for Experimental Track users sometime this week. If you want to sign up for the Concept program, you can find more information here.