Yes, shey, it is determined by metadata. You can change this specific metadata in a mobi with
Java Mobi Metaeditor:
https://www.mobileread.com/forums/sho...d.php?t=165729. The first page of the topic shows what you should do to make Kindle see a book as a "book" or as a "personal document". I have no clue on magazines, hope someone can help.
(If you don't like the Java tool, Mobi2Mobi does the same thing:
https://www.mobileread.com/forums/showthread.php?t=21763. AFAIK, Calibre can't do this.)