The first reply said it best - once you add coral to the tank, the focus shifts and you start tailoring parameters to meet that coral's needs - therefore, when you add coral, you have the beginnings of a reef. One does not have to keep a certain percentage of stony corals to have a reef, that makes no sense to me. What would you call a tank filled to the brim with zoas, ricordia and leathers, if not a reef? A stand of Sarcophyton leathers in the wild would be considered a reef area, even if there were no hard corals in sight.
IMO:
Fishless reef: rock (live, base or both), coral (any type/number) and SW. Possibly also snails, crabs, shrimp, hermits, reef lobsters, anemones, other inverts
Reef: rock (live, base or both), coral (any type/number), fish and SW. May also contain any of the above inverts.
FOWLR: rock (live), fish and SW. No corals, but may have inverts like shrimp, snails etc. though many don't (FOWLRs are often predator tanks)
FO: fish, SW, possibly artificial decorations or bleached coral skeletons a la 1980's. No corals, no live rock.