The answer is hair length. It doesn't tell you how the hair behaves, so it's not part of picking the right product.
Density, porosity, and texture all describe how hair actually behaves — how much of it there is per square inch, how it absorbs and holds moisture, and how it falls and feels — and that behavior is exactly what drives product choice. Length just tells you how much hair there is, not how it'll respond, which is why it's the odd one out here.
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