Quantities are where drawings, specification and model meet money. Each specified material or grade becomes its own quantity line, measured the way the measurement rules say, and the model can only produce it if its elements carry the right properties.
A quantity is a promise about money. The estimator multiplies it by a rate, the contractor is paid by it, and the client budgets on it. It comes from three places at once: the drawings (how big), the specification (what material, and how it's measured) and, more and more, the model.
Take concrete. The specification gives each class (say C32/40 for car park slabs, C28/35 elsewhere) and how concrete is measured. The drawings give the sizes. The model can then produce volumes, but only if:
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