Questions about my W-2

Hi all! I was reviewing my 2021 W-2 and just had a quick question.  I read the "Instructions" section on the second page of my W-2 in hopes to gain better clarification about what Box 12 is for, but the more I read it the more I get confused.  Would you be able to explain to me in just a few sentences / dumb it down what these boxes really mean?  I'm not seeking tax advice, I just kind of don't know what these boxes mean....

 

12a: there is a letter code "C"

12b: there is a letter code "D"

12c: there is a letter code "W"

12d: there is a letter code "DD"

 

Also, I received two W-2's for 2021, since I had two different jobs (switched jobs in the middle of 2021). For one of my W-2's, the Box 15 states "TX" but for the other W-2, the Box 15 section is blank.  Is that okay?  Just making sure that a blank Box 15 is okay....

 

 

I really appreciate any help you can provide! I'm not seeking tax/professional advice, I guess I'm just seeking for better understanding on the boxes.... thank you!