[Samsung Galaxy Note 9 Questions & Answers] IT_Architect:
Observations:

For any particular build generation:

1. Between the U and U1, none of the firmware files are the same, not AP, BL, CP or CSC.

2. Between the U firmwares, only the USERDATA changes. The others have the same name and SHA-512.

3. Between the U1 firmwares, ALL of the files have the same and SHA-512. This indicates SamMobile's different zip files for XAA, VZW, TMB, TMK, etc. are an "exercise". U1s are sold by non-carrier outlets who have no idea where the phone will be used.

4. Flash file size differences between the U and U1 are as follows:

AP - The Unlocked U1 flash is more than a gig larger than for the carrier U flash.

BL - They are very close to same size with the carrier version being 41K larger.

CP - They are very close to the same size with the carrier version being 327K larger.

CSC - The carrier U flash is about 20 megs larger than the Unlocked U1.

USERDATA - T-Mobile is 1.35 GB, Verizon: 1.76 GB. The US UNLOCKED U1 doesn't have a USERDATA flash file.

My Theory:

There are many theories swirling about that center on carriers restricting services when you don't buy your phone from them. My theory is much simpler. With all of the U carrier firmware, you will see a gig plus USERDATA flash file. The U1's firmware doesn't have a USERDATA flash file, but the U1's AP flash file is a gig plus larger than U's AP flash file. I also work in the UNIX world. Applications that are installed at the same time you install the operating system will be installed in the operating system area. If you install those same apps later, they will be installed in the user software area. My theory about the Note 9's firmware is that the U1 Firmware is Android+Samsung-specific software+Samsung specific hardware drivers+standards-based wireless network drivers+Samsung basic apps. Samsung gives wireless providers the same with standards-based wireless network drivers+Samsung basic apps in separate install. This could explain how the gig plus moved from the U1's APP to the U's USERDATA. It would also enable a consistent user interface for the same service across all wireless providers who may be providing the service using entirely different methods. This would explain why running the carrier-specific the U firmware vendors all support capabilities such as Wi-Fi calling and Video Calling, while when the same phones are running US Unlocked firmware, only some do. That would mean those with the services available have deployed them in a standards-compliant way. For instanced, for T-Mobile, Voice over LTE(VoLTE) and Voice over Wi-Fi(VoWiFi) work but USCC roaming doesn't. On Verizon, Voice over LTE(VoLTE) and Carrier Aggregation works, but NOT Voice over Wi-Fi(VoWiFi). With AT&T, neither work. So that likely means the Verizon does not support VoWiFi on their network while AT&T doesn't support VoLTE nor VoWiFi, and have proprietary implementations on their network that are only available if your flash their carrier-specific firmware. This would also explain why when a phone is running the US Unlocked U1 software with a Verizon SIM installed you will not have Wi-Fi calling available, and when you simply swap to the T-Mobile SIM, it will magically show up. This seems to me to simply be the U1 firmware responding to what it finds. It hasn't been that long since our phones would only work on one network. It hasn't been that long since the critical mass existed to migrate to the new LTE standard. The T-Mobile build-out only increased rapidly, and Sprint may not even be finished rolling out VoLTE yet, thus much less likely to have non-compliant systems in place. You could extend this reasoning to other wireless technologies as well. It also makes sense why the mechanism to prevent firmware upgrades to not be present in the U carrier-specific firmware, while it is present in the the standards-compliant U1 firmware. The U1 path adds support for new standards as it goes. The U path must replace proprietary methods for advantage or regulatory reasons. If this theory is correct, you would be better off with U1 firmware when traveling abroad. The name US Unlocked may have little to do with US, and a lot more to do with being standards-based, which is more likely what you would encounter in other countries where widespread deployment occurred later. The SM-N960U* already supports more bands than any phone I've found, and with US Unlocked U1 firmware, it may be the best world phone as well.

Why I believe this:

In an environment of democracy and capitalism, companies are free to make decisions that benefit themselves the most, and those who are stockholders appoint CEOs who know how to make that happen. They would get fired for not using other people's equipment and networks for free if they could. If they could make more money by getting rid of non-VoLTE devices to free up bands, they will, and the FCC is forcing it to happen. The fact that VoLTE is not controllable by the user interface, does not indicate it is not being used. Why would you give a user the option control VoLTE, Carrier Aggregation, etc.? Voice Over Wi-Fi VoWiFi employs SIP/VOIP technology, which predates smart phones. Towers and equipment are often not owned by the wireless providers because they are co-location sites that must be approved by local authorities. Standards-based equipment means it would be cheaper, you need less of it, you have more options with regard to inter-carrier agreements of which some are mandated by the FCC. Moreover, investments in non-standard methods and equipment devalues the company, and even determines how much they can leverage money. Holding out on people who do not buy their phones from them doesn't make sense because they are not denying these services to BYOD pre-pay users, or anyone who flashes their phones to their firmware. The FCC already forces them to unlock their phones before they are paid for, so it doesn't make sense to put themselves at a competitive disadvantage when it comes to acquiring users from other carriers. If carrier salesmen posts are true, they don't make much on phones anyway, and phones are more a vehicle to sell their services and accessories. They know that you know they don't have you over a barrel anymore and implementing interoperability is their new reality, and the reality of any maturing technology.

Surprises and Mysteries:

After I paid my premium membership for a month and went to SamMobile to download US Unlocked U1 firmware, I was surprised to find files for all different wireless network providers. Puzzled, I downloaded them all. SamMobile shows TMK as USA, so believing that I was getting US Unlocked, I downloaded it and flashed it. Booting up I got the Samsung Logo, which also made sense. I learned later that SamMobile has things mislabeled. TMK is MetroPCS, and XAA is the US Unlocked I should have flashed. However, then I noticed after the U1 files were unzipped that the flash files all had the same name. Then I tested their SHA-512 numbers and they were exactly the same, which means they are the same files. So, if they are the same files, why do they list the different wireless providers and put them in separate zip files with the carrier's extension on them? There isn't even an expectation of a difference because when they sell a U1 phone, they don't know which network it will be used on. What I do know is the Phone INFO app shows:

Firmware's CSC Code: XAA (I never flashed the XAA zip file. I flashed the TMK zip file, but since I found the flash files were the same for every U1 firmware zip file, seeing the US Unlocked code of XAA was not shocking.)

Active CSC Code: VZW (SIM installed)