What's the difference between the 65R 65T 65S etc on the tyre sizes?

Does it really matter what letter I'm after?

Also I'm guessing recommended tkc80 size is 130/80/17 (65R)

Taa

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XT660z......What a machine!!

Current mods:- Yamaha top box, Yamaha hanguards, Heated grips, High front fender, Aux lighting front/rear.