As Nexus 6 owners well know, Android 5.1 / LMY47D just started rolling out to devices recently. It appears, however, that Motorola has an even more recent firmware available, build LMY47E. Further, the official filename has "VRZ" in the title, hinting that this newer build may ship on devices that are sold by Verizon. We won't know for sure until Big Red starts sending the devices out next week.
There is a lot of historical precedent for this. Past builds such as GRI54 (Nexus S), GTJ61 (Nexus S), IMM30B (Galaxy Nexus), JRO03S (Nexus 7 2012 Wi-Fi), JOP40F (Nexus 10), JDQ39B (Nexus 7 2012 Wi-Fi/LTE), and JDQ39E (Nexus 4) are all examples of pre-installed firmware that shipped on later-manufactured runs of Nexus devices, but were never released to older devices via OTA or factory images.
— Moto Firmware (@MotoFirmware) March 13, 2015
This build is only available as RSD images right now, though, so you'll need to download Motorola's RSD Lite application, open the ZIP file, and flash it that way. I cannot stress enough that RSD Lite is to Motorola what Odin is to Samsung. It's a very powerful OEM flash tool that you can do major damage with. Make sure you know what you're doing because I promise you that your phone will walk straight into Mordor if you start checking off boxes that you don't fully understand. There may also be a way to flash these using the more traditional fastboot method, but the system image is broken up into chunks, which may make that difficult.
As for what actually changed between LMY47D and LMY47E, we have no idea at this point since the latter doesn't exist in AOSP yet. We know it does have a slightly newer radio, though. In the end, this build probably won't give you any tangible benefit over LMY47D and probably isn't worth resetting your phone for. If you like to be on the absolute bleeding edge, though, have a ball.