Slip question

Do all slips for ballgowns have hoops? What are the different amounts of fullness available? I'm looking at bridesave.com right now but I'm still confused! I think the lady at Burton's said to get a slip that has three tiers? Am I making that up?

I have a ball gown and my slip is extra full but does not have hoops. It is all tulle and crinoline. Here's a pic of the same one I have:


I bought it on ebay from a vendor called Bridalpetticoats.

thanks