I thought it was well known, that's why nobody answered. I am actually sure it was discussed here before. It's not new at all.
Anyway it just an other restricted class rating to allow horses who reached a certain class but then got stuck there and never win, to race again in a lower class race where they have a chance of winning again.
Look up one of the race club's website, maybe Victoria, for details. I am actually sure someone posted the class value of these RB numbers, as well.
If I can find it I post it for you.
<62 = Class 1
>62 >= 68 = Class 2
>68 <= 72 = Class 3
>72 <= 78 = Class 4
>78 <= 89 = Class 5
>89 = Class 6
Good luck