That's how I thought about it when reading. I kept wondering whether I was missing something, but: the things to be modeled by programs have a continuum of comp

That's how I thought about it when reading. I kept wondering whether I was missing something, but: the things to be modeled by programs have a continuum of complexities, but there are only a small number of language constructs to use for modeling, which are each more/less suitable based on the complexity of the thing to be modeled (for one thing). The 'language constructs' from simple to complex are something like: enum, struct, class. It sounds like the author wants something that's between struct and class, though personally, I don't see why doesn't just use a class/objects.