By default auto tag process searches for all tags (artist, title, lyrics, cover art, album, genre, year, album artist, track number) but it will not overwrite existing tags in your file.
That means that if tags already exist in your file auto tagger will not change it although your current tag might be correct or incorrect. If you check “Overwrite existing tag” it will overwrite (replace) the existing tag with the results from the auto tag process.
If you don’t check specific tags (for example Album Artist) it will not be written to ID3 tags at all.
Default volume for normalization is 89.0 dB. You can see it on status line. You can change it to another values then 89.0 dB at Options-Normalization-Target Volume.
It is always good to make undo track volume, before going with another target volume if files are already normalized.
Zortam is using mp3gain (http://mp3gain.sourceforge.net/faq.php) for the normalization process.