Bootloader unlock partitions clarifications

[DJTusheniyTapochek]

In usage README, it is said that following partitions need to be deleted after bootloader is unlocked:

• userdata
• metadata
• md_udc
• cache (for some reason, this partition is not mentioned below, where information about bootloader unlock usage is duplicated)

I wonder why exactly all these are needed. metadata looks like empty (filled with zeroes) from the beginning, erasing it makes no sense, because nothing happens. cache seems like cache obviously, and if I understand correctly, it can also be erased using custom recoveries as part of trying to fix some issues, but I never erased it when unlocking bootloader, and it worked correctly. userdata is deleted as part of factory reset performed automatically during bootloader unlock using official methods, but some people reported that not erasing it doesn't cause any issues and system boots with data intact but bootloader unlocked.

But md_udc is most mysterious one. What does its name mean at all? I checked various websites, but only found AI hallucinations and speculation inspired by them, including about what is stored in the partition. I eventually extracted its contents compared original partition with one that got restored after being erased, and noticed the difference is in "bootstat/persist.sys.boot.reason", a lot of directories in "apex/sessions" (that don't match between two different devices of same model but different storage size), and "gsi/dsu/avb" with three generic public keys copied from GSI repository, all only present on the original partition. Unfortunately, the devices already got custom ROM running, so maybe it is the real reason for these difference. Is the partition for some OTA update cache, or something else?