When SongKong can match a song but not determine the album it will update song metadata such as song title but not album metadata such as Album Title or Track No regardless of any existing metadata. This means that the song is identified and updated with useful metadata without making any changes that would prevent it being identified as part of the same group on a subsequent run of Fix Songs.
However even modifying only Song Only metadata such as the Song Title or Track Artist could still change values significantly so that there is a difference between these songs and the totally unmatched songs. This difference is more likely when dealing with Classical release since there is not a single standard way that these fields are used with Classical music, for example you may have some songs where the work has been included in the song title and others where only the movement is used. If you disable this option then it will only update fields that are currently empty and this may better preserve consistency if some songs in an album are matched song only and some are not matched at all. The disadvantage of disabling this option is this can prevent SongKong replacing existing incorrect metadata with correct metadata.