Once upon a time, long ago, multiple submissions were verboten. They were considered to be aggressive and rude. Each publisher felt that they were entitled to an exclusive look and that they could take all the time they wanted.
[Pause here for me to stop gagging}
Imagine that. It could easily take two years to sell a book, and publishers thought that was just fine. Agents did not.
So agents began to send out multiple submissions whether the publishers liked it or not. That way you could sell a book in a matter of days or maybe a few months. It was a hell of a lot faster and more expedient. Today it is the norm. Exclusive submissions can happen, usually by prior arrangement, but they are the exception to the rule.
When you do a multiple submission, you hope to get more than one offer. When you do, you have an auction. Here’s how it works.
As soon as I know that more than one is interested, I set a closing. Publishers must offer by that date and time. The time has to be exact, because offers usually come in at the last possible second. Once I can see what’s on the table, I move forward accordingly. There are different ways to do this. But the general idea is to tell everyone what the highest bid was and do another round. This can go on for a while. If the offers are very close together, you can declare “best bids”, in which everyone makes the best offer they can make. The goal is to flush out the highest bidder in a fair and reasonable manner. That sounds simple, but it often isn’t. That is why one of the first question an editor will ask is “What are the rules?” That is a time for me to explain exactly how it’s going to go. I may ask that each bid be a certain amount more than the last. Or I may eliminate the lowest bidder of each round. Or I may accept a pre-empt at any time. That’s when you get an offer so good that you decide to just accept it, shutting the others down. To complicate things even more, the house offering the most money isn’t necessarily the winner. The author may like another house better or have a better rapport with the editor. That doesn’t happen often, because the house with the most passion for the project usually offers the most money.
No matter how well you have it planned out, something can always go wrong. We have to expect the unexpected and stay focused. If a lot of bidders are involved, you never know what might happen. They ask what the rules are, and then they break them. But I have found that if I am fair and clear from the outset, it usually goes well.