It is within the markup category as it is not just text but contains formatting syntax to present the text. Most people don't think of it as one (nor would most people probably think of a OMdoc file as one) but, really, technically they would certainly fall in that category.
Here's some information for you:
http://msdn2.micr...7(office.10).aspxIn order to prevent confusion this would TYPICALLY be called a "document markup language." This being a "Web Development" section of the forum and RTF having not really been created for the web it might not be best suited in this section but, frankly, who cares? If someone were to present a question about RTF in this section someone would probably answer it.
Here is a list that shows some markup languages:
http://en.wikiped..._markup_languagesI don't think it is complete but, bah... Bugger it.