The type property is a doctype specific category.
While Subject, Categories, and Tags are typically supported by all doctypes, the Type property refers to a document type specific type. Therefore there is typically a specific doctype type for that type (like the Role Type), while the documents for subjects, categories, and tags are shared by all doctypes.
Example Type on Role Doctype
There is a Role Type that allows to group roles by their type. There may be administrator roles and user roles. The type
Administrator and the type
User will not be shared with any other document types.
Do not be confused that there is a Tag Type to group tags. You can also tag documents of doctype Tag with the Tags property. The Type property of a tag document will point to a document of type Tag Type.