Store on the one-page lister is for ebay Store only, i.e. posting Buy-It-Now (or Best Offer) items to your Store. That option is only available to ebay Store subscribers. If you don't have an ebay Store, then this option isn't one you can use. Store has it's own separate list of fees including the Store subscription fee and you list items (including quantity items) for 30 days or good until cancelled.
Auction (and Fixed Price) items or lots (like Dutch auctions) will also appear in an ebay Store, but have more visibility to search, etc., like any other auction item. They follow the standard 1,3,5,7,or 10 day listing periods and those standard set of fees.
That's probably a little to simplistic, but hopefully gives enough distinction to Store vs. non-Store to help get you started.
Best place to research ebay Store is on ebay's website. Good links are found in the "Site Map" link that's visible at upper right on most ebay pages including the home page.
Danno