The two numbers that actually define a sprinkler system's hazard coverage — and why a system that looks adequate on one of them can still be under-designed.
Every NFPA 13 sprinkler hydraulic design is built around a pair of numbers, not one. The design density is a rate — how much water, per square foot, the system must be able to deliver to the protected area for the occupancy's hazard classification. The design areais an extent — the specific floor area over which the system must be able to sustain that rate everywhere at once, simultaneously, because that's the assumed worst-case footprint a real fire could put into operation. Neither number means much without the other. A rate says nothing about how far it has to reach; an area says nothing about how hard the water has to work once it gets there. Sprinkler hydraulic design exists entirely at the intersection of the two.