Meaning of Samsung bootloader codenames? I realized that you cannot install firmware with older bootloader version than the one in the device.
To do things more correctly and safely, I want more fully understand the bootloader codename meaning.
Here are diffrent bootloader codenames, that appear in different three Build Numbers.


Build Number: G965FXXU7DTB4 
Bootloader Codename: U7D

Build Number: G965U1UES7DTB2 
Bootloader Codename: S7D

Build Number: G965USQU6CSI2 
Bootloader Codename: U6C



I understand  that the digit between the two letter indicated the bootloader version. 
Therefore, the bootloader version is the same for U7D and S7D and the U6C is older.
However, it is not the same codename.
What is the difference between U7D and S7D?
I'd love to know what the diffrent letters means?
What is the difference between U7D and S7D?

First, the difference between U and S:

The x is either a U or a S. U updates tend to have actual updates and/or new features. Or simply bug fixes. S updates tend to be strictly security-related.

Secondly, the meaning of 7:

This is the bootloader number/fuse. What you need to know: You cannot rollback to a version that has a bootloader fuse lower than the version you currently have. As such, if you have a 3 BL fuse, you cannot go back to a 2 BL fuse. But you can go forward to a 4 BL.

Finally, the meaning of D

is the software upgrade version. The first version is always A. When there is a major update, such as a feature update or an Android upgrade, this letter goes up. Note that for official beta versions, the letter is always Z.

All the above quotes are from How to read build versions.