The difference between 85RD and 106FF is the level of cut, gloss level is the same according to Menzerna.
I have used a couple of litres (quarts) of each and find each has its uses.
1. On cars in poor condition with lots of swirls I use a rotary with PO85RD3.01 on either a yellow or orange CCS pad then follow up with PO85RD on my Festool Random orbital to ensure no holograms etc.....I can then go straight to my LSP.
2. On cars in reasonable to good condition IE: Light swirls, I use a rotary with 106FF on an orange or white pad, then change pads to a white or grey to finish off with (still using 106FF). Then I am LSP ready.
In summary 106FF is a one step product for some cars, whereas 85RD is really a final polish to be used after a more aggressive first step. Or on cars that are already in very good condition.
I also find that 106FF dusts less than the others and is easier to remove the residue in our humid weather.
Cheers
Dave.