First, they are not XML tags.
They are a hybrid of the HTML mark-up tag set that are required by the EPUB standard.
Just because they have pointy brackets and similar names to XML tags, they are not XML. XML = extensible mark-up language, which means that anyone can make up their own tag names and rules of usage.
That's the opposite in HTML, EPUB, and PDF/UA where you must use a given set of pre-defined tags and use them correctly as prescribed by the rules (aka, guildlines, standards).
You control which tag ends up in the exported EPUB in the Export Tagging section of your paragraph style definition. See below.

This is a very elementary question about how EPUB works. Lynda.com/Linked-In Learning has excellent online training in the basics of EPUB. And you can sign up for a month's worth of free access.