This is used to have more the one «Shopping Cart» on a single table. For example, one is an actioal Shopping Cart another is a Wish List or Quotation.
Prifex is an adition texttual string added to the cockie.
To filter catalog page (Menu Item with Extar Search Catalog type or ExtraSearch Catalog Module), look for the box «Shopping Cart Prefix» and type desired prefix there.
Empty prefix is ok.