Item Sets Tree
Item Sets Tree is a module for Omeka S which allows to organize item sets in a tree.
See general end user documentation for Installing a module
How it works
Once the module is installed, you can define a parent item set when adding or editing an item set.
You can visualize the whole tree by clicking on the menu entry in the left sidebar.
The tree is also accessible in public sites. You can add a navigation link to it in Sites » Navigation