Thanks for your reply Florian (and ohrenkino)
I understand how to use tag sources and how to add custom ones.
I would like to create my own by starting with the original which works 95% for me, and just modify a few fields I want to be different. I also looked at other Discogs custom scripts, but they are either not updated/maintained, and/or very complex to get started with and debug.
This recent post Discogs Release ID - Bundle with Field Composer and Performer suggested to me that at least some people have access to the original src to start developing on their own customization or am I mistaken?