Meaning of Samsung bootloader codenames?
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.