Welcome!
Some quick replies:
- Picard itself only pulls tags from the MusicBrainz database.
- If you are using ‘scan’ then Picard will ignoring your existing tags, and scan the actual music file, and try match it to the AcoustID database. The same song/AcoustID can be on multiple albums, which is why this method will throw up a lot of compilations etc. Personally I only use scan as a last resort, but then again some people use it a lot.
- If you have good tags already, then you probably want to use ‘cluster’ > ‘lookup’ workflow, as lookup searches the database using the existing tags.
By the way, Picard will overwrite existing tags, unless you specifically tell it not to!
So please be careful, and test it on a copy of your files first.
Let us know if you have any other questions!
Cheers