The difference between a Standing Offer and a Listing
Listings are available to purchase immediately and their availability is known, and a clear flighting period defined and they have a price (which may change if these are auctions or decreasing price listings.)
A Standing Offer is not available for purchase immediately - a potential advertiser much first enquire about the availability of this space for a particular period (as well as detail what they intend to advertise) and a Media Owner then has the ability to respond with a quote. If the interested advertiser accepts this quote, then fulfillment proceeds in the same way as is the case when a buyer purchases a Listing: they make payment, upload artwork, this is then subsequently approved by the Media Owner and ultimately flighted.
It is worthwhile noting the both Listings and Standing Offers can have multiple items included (Packages).