How do I hide categories and make them visible?

Hide a category: 

1. Right-click on a category and select Hide category.
2. The category is now hidden.

Make a category visible:

This is only possible when the remark Some categories are hidden can been seen at the bottom of the tree structure.

1. Right-click under the final category in the tree structure and select Show hidden categories.
2. The categories are now visible again.