The Category Tree is a visual representation that allows you to see the full setup of all your parent and child categories within the PIM platform.
This Tree is especially helpful for viewing categories as they are setup in the platform to confirm you have added them to the correct parent category or finding a category code. The Tree is also really helpful when working on your commerce site.
If a change needs to be made, simply click on the name of the category you want to update.
The Details tab will appear so you can make the desired adjustments, like what parent category this category is associated with.
You can then also see the products associated with that category by viewing the Products tab. From that screen, you can adjust individual products or do a mass import.