Does anyone know where is this Account number setting comes from? This screenshot is from the GL Impact Tab from a Credit Memo created from a Return auth/Invoice, I need to change that account for something else in some cases.

Perhaps you mean something like this? https://blog.prolecto.com/2015/09/27/netsuite-up-close-custom-gl-lines-plug-in-to-reclass-general-ledger-postings/

I imagine the account is coming from here (on item record):

Yeah, but the thing is that the GL account should only be changed for some cases. The options seem to be either using a special RMA item linked to that GL account or a GL plugin as @ec pointed out.