What constitutes the "missing middle"?

[Docere]

What's defined as "missing middle" can range from everything "in between" detached houses and high rise buildings. The more "purist" side would probably come pretty close to the Statcan dwelling classification of "Apartment less than 5 storeys."

I do wonder if "5+ storeys" is too low a threshold for "big apartment", as it's often used as synonymous with high rise in these discussions. But that's what Statcan uses, so it's hard to distinguish a 7 storey building and a 25 storey building say.

Anyway...let's discuss.