I just looked up. All are ranked. In the Navy, enlisted are rated. Why the USN has this I have no idea. Additionally, the USN gives the enlisted a rating. The rating specifies his field of expertise.
For example, a PO First Class would have a rate of E-6. His rating might be ‘Electronics Technician’.
In the USAF and Army, it’s called grade. It is possible to have equivalent grades and different ranks. It also makes it easier to figure ranks structure between the services. Navy ranks confuse the rest of the world. :)