1. Undo data is used to rollback from fork? For example: was 500000 blocks, now incoming block 500001, but is fork and incoming new 500001 and 500002.
We use 500001-rev to undo transactions?
Yes, the -rev files are used to basically rollback to the starting point where the new (longer) chain starts, and then validate the transactions in the blocks. Basically, it reverts all the spent inputs from the "old" chain back into UTXOs (unspent transaction outputs), so it can then determine whether the inputs in the transactions in the "new" blocks are actually valid etc.
2. I use Litecoin client for test because Bitcoin blocks have a lot gigabytes. I don't see *.rev files but probably undo info exists. Undo information is in *.dat?, also in new Bitcoin client?.
I haven't used Litecoin Core in about 2 years... but my leftover litecoin data directory from 2018 still has rev****.dat files in the blocks directory. I installed the latest version 0.18.1... and after it had "updated the UTXO database", the rev****.dat files still exist. So, I'm not sure why you don't have these files?
![Huh](https://bitcointalk.org/Smileys/default/huh.gif)
Are you running in "pruned" mode? Is the highlighted box in "Settings -> Options -> Main" checked or unchecked?
![Huh](https://bitcointalk.org/Smileys/default/huh.gif)
![](https://ip.bitcointalk.org/?u=https%3A%2F%2Ftalkimg.com%2Fimages%2F2023%2F11%2F15%2FzvZrJ.png&t=665&c=uWWykBGjHV62Og)