In other words, an assignment clause allows the buyer of a home to sell the place before they take possession of it. In Ontario, assignments are more common in pre-built homes and condos than on re-sale properties, but they are possible on any type of trade.
Writing your own contracts is perfectly possible, and legal. But it's also an incredibly bad idea. There's two reasons for this: Property law is complicated. Because it's such a fundamental part of legislation, it's often lots and lots of different laws layered on top of each other.