The category tree
The left pane of the Browse tab shows the catalog's categories, nested.
What it shows
The tree shows the catalog's categories. Expanding a node reveals its subcategories, and selecting one fills the route list beside it.
Select a category
Selecting a category drives the route list next to it. Nothing is downloaded until a route is opened.
Add, rename, delete a category
The buttons under the tree — Add, Rename and Delete — create a category, rename the selected one, or delete it. These need a login — see The route catalog.
What "Categories for routes" means
Categories for routes: is the label above the tree, naming the tree's purpose.