h1 is used to semantically mark chapter headers. During a conversion, this gets split into different pages.
You can use other header tags (h2,h3,h4,h5,h6) to mark different stuff, and set the xpath to detect however you set it up. Kovid already explained it better, though, so why don't you take a look at the manual, here:
http://manual.calibre-ebook.com/conv...nd-page-breaks