My basic guide is, the denser the pad, the more abrasive the polish... and then the softest i use for waxes, sealants...
I have to use this guide, because im not a hardened detailer, but i like to have a dabble very now and then, and by the time i go to use the pads again, i forget what polish i used on what pad, ha ha!
A black marker helps, you can write on the back of the pad what polish/stage you used it for, so its good for next time....