"Subcategory of" is a predefined property to define that a category is a subcategory of another and is provided by Semantic MediaWiki. This property is pre-deployed (also known as special property) and comes with additional administrative privileges but can be used just like any other user-defined property.
This is the page of special property Subcategory of. It lists all properties using this special property. See the help page Special property "Subcategory of" for further information.