It is just to designate the model. 'XR' is a 4-stroke dirtbike. The 'R' means that it is a dirt-only model.
For example, Honda also had a CR80R. The CR meaning a 2-stroke dirtbike. Once again the R meaning dirt only.
Then there was also an XR600L. You already know that it is a 4 stroke dirt bike, but the L means that it was a street-legal model.
The list goes on... Hope this helps!
