What is tier11, Tier1A, and TierB on a w2 mean?

Where are you seeing that?   And are you sure you are looking at a W-2----or some other tax document?

If it is a W-2, which box of the W-2 are those items in? Are there dollar amounts associated with them? If they are in box 14 on a W-2, you have to ask your employer what they mean.

On a W-2, in Box 14 , Tier 1 and tier 2  (Sometimes Tier I and Tier II)   Refer to SS and Medicare taxes for Railroad employees.   

Tier 1A and Tier B?? (Tier 1B?)   Yeah, you'd have to ask your employer

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.....as a pure guess...IF in box 14 on a Corrected W-2C for 2020? 

Maybe 1)  IF some of SS taxes were deferred for 2020 income...Tier 1a might be the original amounts of SS taxes withheld.

Then the proper deferred amount was paid back from wages during 2021, and the W-2c uses Tier1b label to show the amount that was paid back.

or

Maybe 2)  Same situation,  but employee left RR employment before all was paid back...Tier1a label refers to payback amount during employment in 2021, and Tier 1b refers to amount employee paid back to the company (to forward to SS) after leaving employment.