Air Peace,
On the New Image color ring the R stands for red. The hair will have some red color to it. As far as the RD goes there is only only number with that designation---6RD. I'm waiting for a response from the factory on that. There is no connection between 7 and 17 just different colors. 17 and 18 look very close and it's difficult to tell any difference when you look at them---that's why the factory wants you to send in a hair sample. The difference is so slight that it needs a trained eye to detect the difference. Bottom line: send in a hair sample and let the factory match the color.
Regards,
John