Sitemaps are the way to tell search engines crawlers about the URLs hosted on your website and there role become even more important when pages are dynamically created rather than static HTML files. Thus, they play one of the most critical tool in SEO.

Now, consider a situation where your website is constructed from multiple dynamic systems which are independent. They can construct their own sitemaps. So, you now need to bind all of them into a single entry point so that crawlers can pick them up.

Sitemap Index is a solution here, just like index of a book, which tell us the further references at a single place. You just need to replace the contents of your actual sitemap.xml with the format of sitemap index file as shown below:

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
    <sitemap>
        <loc>http://www.abc.com/products/sitemap.xml</loc>
        <lastmod>2013-08-08</lastmod>
    </sitemap>
    <sitemap>
        <loc>http://www.abc.com/categories/sitemap.xml</loc>
        <lastmod>2013-08-08</lastmod>
    </sitemap>
    <sitemap>
        <loc>http://www.abc.com/services/sitemap.xml</loc>
        <lastmod>2013-08-08</lastmod>
    </sitemap>
    <sitemap>
        <loc>http://www.abc.com/portfolios/sitemap.xml</loc>
        <lastmod>2013-08-08</lastmod>
    </sitemap>
</sitemapindex>

Note: The sitemap urls in sample code above are just dummy.

Reference: http://www.sitemaps.org/protocol.html

Enjoy!!