What is the basic difference between Flared Wire Decks and Step Wire Decks?
Flared or Universal Wire Decks have support channels that flare at the end. The purpose of this design is to allow for compatibility with both Step Beams and Box Beams (Refer to FAQ for Pallet Racking). See photo below: 
Step Wire Decks have square shaped (or V shaped) support channels that are designed to nest into the instep of a step beam. See photo below: 