Ok, I think I'm right in saying that the rays are the lines coming out in the tail and fins. Have borrowed this pic from Bettaman's site as this lovely girl shows them quite well.
Can you see the red lines splitting into I think 4 rays towards the end of her tail?
Please correct me if I'm wrong anyone and if I am, then I'd like to know the answer too
As far as I know, that's right. The tail veins start as one, then split once, and then once again. Personally, I think they look horrible in crowntails, but they can look really nice in half moon.