Property_Item : sub_class_of : Item_Class

This category item is the base of the hierarchy of items that are properties. It is a class, and is therefore identified as such by the connection "Property_Item is_instance_of Item_Class". The absolute requirement to identify items as being "property type" (as opposed to individual or category) is to specify that when they are created. However, most properties will also be identified as such by the existence of either the connection "[property] sub_property_of [another property]" or "[property] is_instance_of [another property class]". In the second of these connection forms "another property class" will be either Property_Item or one of its children.

This relationship is used to connect two items, both representing classes, indicating that the subject item is a sub-class of the object item. For example, the connection "Ships is a SubClass-Of Vehicles" links the two class items, and makes the statement that "Ships" are a more specific kind of "Vehicles". Once this connection is established, the system will behave as if everything that is true about "Vehicles" is also true for "Ships" (although not vice-versa). For example, if another property is defined such that its object items are required to be instances of the "Vehicles" class, then having this connection allows instances of "Ships" to be used as well. But, another property whose objects are required to be "Ships" is not affected, and other "Vehicles" still cannot be used.

This category item is the foundation for the hierarchy of items that represent "classes." A class is an item, some of whose properties make statements about the other items that are "instances" of that class. Wontologies can be constructed where the membership (set of instances) in the class are determined by rules that compare against the other items in the wontology, but more commonly, a class' instance items are explicitly identified using the is_instance_of property. Any item that is to represent a "class" (a category whose child items are "instances" of the class) should be identified as a child of this category or one of this category's other children using the sub_class_of property.