Straightedge tolerance explained?

Obviously the smaller the number the more precise. For example woodpeckers serx aluminum has a tolerance of .0015” per foot while their steel is .0002” per foot.

So 2’ aluminum = .003” 2’ steel = .0004”

How does this manifest itself? Is it as simple as putting it on something truly flat and having a gap, or possible gap of up to .003” or .0004” or am I not understanding something?