Android 7.0 Nougat is the next level of the mobile operating system and at the moment there aren't that many devices which have access to it. Google's Pixel phones have launched with Android 7.1 Nougat and Android 7.0 has been made available officially to Nexus devices, but beyond this many manufacturers have yet to send out OTA updates to their devices to move them off of Marshmallow. While not the final version, Sony has begun to push Android 7.0 to some of its devices as part of the beta program, allowing a few users here and there to get a taste of what the software is like before it's pushed out to all compatible Xperia devices as a stable and final build.
The Xperia beta program is certainly not a new thing, as it's been used to send out various builds of Android Marshmallow and Android Lollipop. With Android 7.0 Nougat, the beta is a little more limited than past versions of the Android software, and Sony has only been sending out the build to one device right now, which is the Xperia X Performance. In addition to this the beta version software is only hitting a handful of devices in various regions so it's not very widespread just yet, and it may not be as Sony could decide to simply leave it to the few that they have already pushed the OTA to for the entirety of the testing period.
For those that have already gotten their hands on the software, it's reported that it includes the October 1st security patch so there's more than just the Android firmware version that's up to date, which is a good thing. Based on the details it seems that Sony has included a handful of Xperia-specific changes with Android 7.0, such as a new version of STAMINA mode, manual shutter speed for the camera, and manual ISO adjustment for the camera. Naturally, the users who have access to this software also have features like split-screen apps, and tweaks to the notification shade. There is no mention of when Sony will push the software out as an OTA to all compatible devices, and there may or may not be more people added to the Android 7.0 Nougat beta.