Sony announced its first devices that carry the number 5 in their names a few weeks ago. Those were mid-range devices with a highlighted focus on their cameras. Still, the company is supposed to launch their flagship lineup later this year, which may include the Xperia Z5, the smaller Xperia Z5 Compact and a larger version which might be called Xperia Z5 Ultra or Xperia Z5+. There are not many details about these phones, but the first two have supposedly been leaked on pictures, showing a familiar design with a redesigned power button which will integrate the fingerprint sensor. Some other rumors indicate that the company will once again use the Snapdragon 810 processor despite the apparent issues it had when handling the temperature in the Xperia Z3+.
Now, the User Agent Profile (UAP) for the models E6833, E6853 and E6883, which belong to variants of the same smartphone. The numbers indicate that we're dealing with the version with a larger screen and while the UAP doesn't provide a lot of information, a few details can be obtained from it. The first detail is that the processor powering up the device will be the MSM8994, which belongs to the Snapdragon 810, which is expected according to previous information. The other detail that can be seen in the document is that the resolution of the display will be 3840 x 2160, which is known as 4K or UHD, and if this turns out to be true, Sony would be one of the first manufacturers to use a display with such resolution.
The company used a display with Full HD resolution in their current flagship, although they used QHD in one of its variants, so probably we could expect higher resolutions in future devices. While we don't know the screen size of the aforementioned device, a 5.5-inch display makes up for an 801 ppi density while a 6-inch display would make 734 ppi, so a very high density in both cases. The included processor could handle such resolution, but the performance or battery life are questionable, as Sony is still promising two days of battery life and yet the batteries keep getting smaller. As for the name, Ultra has always been used to refer to devices with larger screens, while the "plus" was used to name a device with very few upgrades, thus, I'm inclined to believe that they will use the former in the new device.