The proponent is trying to sell us something. This can be a product, a business idea, services, an organizational concept or a combination of these things. The proponent is more commonly called the 'seller'. The prospect, on the other hand, is more commonly called the 'buyer'.
The seller wants to get the maximum amount possible for their proposal, but generally may also set a limit for the least amount they will accept. The least amount they are willing to accept, is known as the seller's 'Reservation Price'. This is the amount where they draw the line, also know as the 'walk away' from the deal point.
The buyer, on the other hand, wants to pay the least amount possible, but may consider a higher amount that they might be prepared to pay as well. The maximum amount they are prepared to pay is also known as the buyer's 'Reservation Price' or 'walk away' from the deal point.
The differences between these respective lows and highs of both the seller and buyer, are their range of expectations. When you have a common ground or overlap between these two different ranges, this is known as ZOPA or the Zone of Possible Agreement.
Of course, common sense dictates that if there is no overlap in the expectation ranges of the seller and buyer, agreement becomes highly unlikely. Similarly, even where ZOPA exists, the agreement might still not materialize,when the parties are unable to agree regardless. The letter 'P' in ZOPA meaning a possible agreement, will more probably occur, but it's not a definite.