Mnet refers to Moodle networking. My main site (www.mydomain,org) is a medium sized Moodle that serves an entire K-12 school district. My second site is dedicated to high school teched ed classes (teched.mydomain.org). Students log in to the main site to gain access to the tech ed site. The click a link. Moodle networking allows me to maintain accounts only on the main site. Tech ed student accounts are auto created when the link is clicked. Mnet also works with the Mahara eportfolio platform. We have a Mahara site networked with our main site too.
Please let me know if you have further questions.