It is both. Instead of thinking of the shaped work as one thing, that is either black or white, predictable or flexible, think of it as patches of black and white. Some things have to be there, and the rest will be filled in.
Imagine you are designing a house. You can specify there must be a kitchen, a bedroom, a bathroom. And even the positions of the walls and the foundation and the load bearing structures. That still leaves a million flexible details, and a vast variety of potential costs (think about all the potential material choices, furnishings inside, etc) but you have total predictability about those key things.
The equivalent in software design is the affordances and primary functions. Those are the load bearing structures.