The functionaly here on the Supportal is actually a plugin developed by us here at Jive for our own Support team, and is not available out of the box. You can read more about our implementation in a few blog posts in this blog: http://www.jivesoftware.com/jivespace/blogs/jivespace
To better serve our customers we have included functionality to automatically follow up on a case after it has been idle for more than 5 days, and then auto close after an additional 3 days of inactivity. Choose No to acknowledge that this case will remain idle for longer than 5 days.
Making cases public allows other customers to learn from the solution of the case. It can also be used to gain feedback from others in the community. Ask our Support Engineers for more info, but we encourage you to make your cases public.