[DaveBlake]

Just as a side note: I'm in the process of adding the ALBUMARTISTS tag. In some cases, this does not help. In my despair I then removed the MUSICBRAINZ tags and then it worked. This surprised me (since I set the provider for album information to "Local information only").

It sounds like you are confusing scanning (of tags embedded into your music files) with scraping (of additional information about artists and albums most of which can not be included in tags e.g. artist biography, or album review).

The ALBUMARTISTS tag is about helping Kodi deduce the individual album artist names from the ALBUMARTIST tag, to match up with musicbrainz album artist ID tags. Look closely at the cases where you say this "does not help". Do you have a different number of musicbrainz ids to artist names? Is it actually an ARTIST and ARTISTS tag issue? Since the tagging from Picard should have been OK, so I have to conclude that you probably edited the tagging in some odd way. Removing mbids could mean that subsequent scraping will be less accurate, and having tagged with Picard it seems a shame, but it is up to you.

Or it could be that you had jumbled names and mbids in the library from older tagging, and that update library alone working through the changed tags is not unscambling it. I don't know as you don't mention what is wrong exactly. Probably worth removing the source(s), cleaning the db (from settings) and adding the source(s) again so it all scans into a clean libaray. This will however lose your last played, and playcount data.

There could be two explanations: some of the files got their size or date changed when you tagged them and some did not (so Kodi didn't re-scan them) or else the MB tags overrode the ALBUMARTISTS. Tags are presumably 'local information' whether 'you' wrote them or Picard wrote them.

No, very wrong.

The whole business of how Kodi merges on-line information with tags is a mystery. I haven't been able to see any difference between 'Prefer online information' on or off. Certainly, Kodi scrapes less data from MB than Picard does.

You are also confusing scanning with scraping.
The music library is populated by scanning tag data embedded in the music files. It takes all the tracks with the same album tag and creates an album entry in the library, likewise with artists. But some details about the albums can be determined from tags too, e.g. year or genre, by taking the common song files values. Details about artists e.g. date of birth or formation, biog etc. and other things about albums can not be derrived from tags, but can be scraped from online sources or from local NFO files.

If you set the scraper settings as provider for album information to "Local information only" then that means don't look online when scraping additional album information, just look at album.NFO file alongside the music files for that album. The other scraper settings are selecting what online source to use for certain data.

For the additional artist and album data that Kodi can scrape see http://kodi.wiki/view/Music_tagging#Data_Kodi_Scrapes

Note that for albums genre, year, (record) label and compilation could also have already be determined from tags. 'Prefer online information' enabled means that the online value will replace that was derrived from tags. Of course if you use Picard to tag, and then later scrape additional information from Musicbrainz then you will get the same year, (record) label and compilation. For genre do you want the ones you choose to tag with, or do you want then overwritten by the wisdom of crowds from an online source (I don't think MB give genre values, but TADB do)?

If you did not tag with Picard (or something else that adds Musicbrainz id tags), or have deleted those tags, then the scraper just uses album title and artist name to try identify the artist and album. This is fallable, the scraper can return the wrong "Beethoven" "Symphony No. 5" release, it can even get the wrong "Greatest Hits" or other albums where names alone do not identify release. If you have 'Prefer online information' enabled and it finds the wrong album, then it will replace the artists associated with that album with the ones it found.

Having had scarping with 'Prefer online information' enabled make a total mess of my classical music, I would say that it is not a safe option to use unless you have mbid tags in all your music files. But for some people (with only popular musical tastes) that made a mess of their genre tags (and prefer the mess made the crowd), or did not tag year, I guess it may have a use.

Tag songs files accurately, include mbid tags to help separate artists with the same name, and if you also want to later scrape additional information about artists and albums. Scan them into the Kodi music library, checking that the tagging has created accurate results. Then, if you like, scrape additional artist and album information and artwork using the online sources that you choose (but 'Prefer online information' disabled, as frankly it is just a liability not a help, and you did all that good tagging).