The last two parameters of an auction, whether it is a native sale or purchase, are optional protection parameters which will create a "bid" auction. Limit Price defines a price at which the underlying must currently be at in order for the auction to execute. This is a bidding process. A purchaser becomes a bidder on the auction. When a bid is placed, an oracle call is made. If the returned spot price from the oracle satisfies the protection limit, the auction will execute, and then succeed. If the price returned from the oracle violates the protection limit price, the bid will fail. The bidder is refunded her bid, and the auction continues.