Some feedback for this DOC page - https://retropie.org.uk/docs/Validating%2C-Rebuilding%2C-and-Filtering-ROM-Collections/

[bollwerk]

Now that I've rebuilt a few older rom MAME rom sets from the (ever evolving) current set, I wanted to offer a suggestion to clarify a little bit how to make this process easier.

I've found it very helpful to have a single folder called "Current MAME", with the following subfolders:
MAME 0.xxx CHDs (merged)
MAME 0.xxx Rollback CHDs
MAME 0.xxx Rollback ROMs
MAME 0.xxx ROMs (bios-devices)
MAME 0.xxx ROMs (non-merged) - you could optionally use the merged or split versions here if you want

Then, when you do the Rebuild process, you can just point it to the "Current MAME" folder for the source, and leave the box checked to look in subfolders.

I know it's mentioned a little later in the page, but it might be good to make it more clear that you usually need the rollback folders if you're going to create an older set from a newer set.