There is a "source" column in both the isrc and the iswc table. Reading the comments in MBS-2885, for ISWCs it seems it has no meaning at all, and it was just added because it was on the ISRC table. All ISWC sources are NULL. For ISRCs, it looks like it probably was intended to indicate sources other than MB itself if the data was imported from a third party, but we never used it. All ISRC sources are either NULL or 0 (defined in the code as "$SOURCE_MUSICBRAINZ"). We don't seem to use either source value anywhere - there's a source method for both code types but it's not called.