Limiting Browse Item Sets in Navigation

I’m new to Omeka S, and I’m having trouble figuring out my navigation settings. I have multiple item sets on my site and would like to be able to browse them separately. For example, I have four item sets I’d like to browse under a “Search Documents” tab and three different sets under a “Reference” tab. However, when I choose to browse by item sets, it seems to be all or nothing. Is there a way to separate them?