In a photo archive, categories play a crucial role in organizing by Theme: Group photos based on common themes (e.g., family, travel, nature).
This is not unlike a file system and follows the same format of folders:, for example:
A Category can contain sets of images and other sub-categories. So you can have the following categories/sub-categories:
Animals category
Four-legged category
Two-legged category
Winged category
Winged category
British birds category
Birds of prey category
So in the Category items you will have the following:
Animals
Animals\Four-legged
Animals\Two-legged
Animals\Winged
Animals\Winged\British birds
Animals\Winged\Birds of prey
Each of these you can include into an images metadata as the category the image belongs.