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Take a look at this tutorial: [link not permitted]
It explains how autotiles are basically made up of mini tiles that are a fourth of the normal size and how the engine puts these together. The tutorial is for 2x2 autotiles and RPGmaker VX/Ace but the same principles should apply to 3x3 and RpgXP.
Edit: I'm not allowed to post links apparently but google for 'anatomy of an autotile tutorial' and you'll find it