In creationism you start out with nothing, or zero, creatio ex nihilo. So the constraint is basically nothing.
A thing is designed if it exists as a whole in the future first. For example there is a likelyhood of fourlegged animals in the universe, from the start of the universe. So there exists this design of basic forms in the universe still, and the design was made at the start of the universe. Paley refers to classes of organisms.
If a thing is formed by many decisions, and the whole is not considered in each of those decisions, then it was not designed. For example if I add bits to a thing, and my options are just between different bits, then i am not designing the thing. But if I add bits to a thing and my options are between different whole things, then I am designing the thing.
That is mainly a science of history. But there is also another meaning of design, which is based on the order of the thing itself. For example in terms of universal language, peoples language has much similar order as DNA does. And so for everything we can look at what order it has.