Okay, so here is the UNOFFICIAL Explanation as Verizon has not provided a complete or public acknowledgement of what the differences are between the two versions or even why. Fortunately, the Development community has played a HUGE role in classifying all of this. Basically there are 2 Updates for the XT926 Razr HD/Maxx HD, the reason for this is there are 2 Versions of JB 4.1.2 which the first is the 9.16.6/MR2 which applies to the largest percentage of XT926 owners (this was the original update of 4.1.2 coming from 4.1.1), the 9.16.9/MR4 was an update that was briefly released (this upgraded MR2 to MR4). If you haven’t taken the update yet, go into Settings> About phone> System version if you have the matching below are the correct updates that should be taken.
1) 9.16.6/MR2 = 9.18.79
2) 9.16.9/MR4 = 9.20.1
Couple of other things, I should mention, with special thanks to Hashcode for originally pointing it out, these updates contained a NEW Radio.
1) VANQUISH_BP_100730.081.64.09P = OLD RADIO
2) VANQUISH_BP_100718.104.22.168P = NEW RADIO
If you are unsure of where the Baseband appears, to into Settings> About phone> Baseband version appears below Android version, if you view the screen shots in POST #13 that is the OLD RADIO and the screen shot in POST #16 is the NEW RADIO.
For those with ROOT, it is still recommended not to take the update until Development versions have been created and released that are SAFE. If you must it is at your own risk, as you know. Those with ROOT ONLY = NO UNLOCKED BOOTLOADER, MUST USE OTA ROOTKEEPER, if ROOT is lost there is NO way to Downgrade. If those that have ROOT = UNLOCKED BOOTLOADER, (still need to use OTA ROOTKEEPER) if ROOT is lost THERE ARE WAYS TO DOWNGRADE, and RESTORE ROOT. Keep in mind if you are not Unlocked already, have not taken the update, this maybe the last chance at gaining an UNLOCKED BOOTLOADER on this device.