Some people buy a condo with the builder during the pre-construction period, so before the condo is even built. The interested buyer will have to sign a pre-construction purchase contract. Once the condo has been built, the buyer will go through an interim occupancy stage where they will simply move into the condo unit and will end up paying the builder an estimated amount which is supposed to include condo fees, taxes, and mortgage. Now, the buyer does not own the condo yet because they are still paying the fees.
However, once the condominium has gone through the necessary legal work, inspections and is officially registered and recognized, then the ownership finally moves to the buyers, and the mortgage payment is established. This entire time period can be stretched out from a few months to a few years, and if during this waiting period till the official closing, the buyer is no longer interested in buying their property and instead want to sell their condo either because they can no longer afford it or because of any other reason, then they will have to opt for selling it, and this is where assignment condos come in.
Since the initial buyer still does not own the property, they are actually selling their pre-purchase contract rather than the property, so this is where the concept of assignment comes in. So, they will have to find someone else that is willing to take on the pre-purchase contract and is willing to consent to all of the details in the contract including the pricing without any form of negotiation, so there is no wiggle room for renegotiation when you are buying an assignment. This is why it is important that you thoroughly go over all of the contract details before agreeing to buy an assignment.