A WSFU isn't a flow rate at all — it's a statistical demand weight. Only Hunter's Curve turns a fixture-unit count into an actual design GPM.
A Water Supply Fixture Unit (WSFU) looks like a flow rate — it's a small number assigned to every fixture, and it feeds directly into a pipe-sizing calculation. That resemblance is misleading. A WSFU is not "how many gallons per minute this fixture draws." It is a demand-weighting factor that accounts for how likely a fixture is to be running at the exact same instant as every other fixture on the system. A toilet can move a real, sometimes large, volume of water in a few seconds — but it does so briefly and infrequently, so its contribution to simultaneous peak demand is rated low. Getting from a building's total WSFU count to an actual design flow rate in gallons per minute requires running that total through Hunter's Curve (or the modern equivalent demand tables) — a specifically nonlinear, diminishing-returns curve, not a fixed GPM-per-fixture-unit multiplier. That's exactly why doubling a building's WSFU count never doubles its required design GPM.