The value of this boolean field will indicate whether or not the Best Offer is part of eBay's Best Offer Beta program, which allows sellers to require immediate payment from the buyer if both the seller and the buyer agree on a Best Offer. A 'true' value indicates that the Best Offer is part of the Best Offer Beta program, and a 'false' value indicates that the Best Offer is a standard Best Offer.
Currently, Immediate Payment for Best Offers will be supported by the Computers & Tablets, Jewelry & Watches, and Art categories, but this feature will get enabled in more categories going forward.
If the Immediate Payment for Best Offers feature is enabled for a listing, and a buyer's Best Offer for the item is accepted by the seller, that buyer will be expected to pay immediately for the item, or that buyer will run the risk of losing the item to another buyer since that item will remain on sale until payment is made.
For more information about the Best Offer Beta feature, see the Best Offer Beta Help Page