This channel behaves a little differently than the specular intensity/color you might be used to. Following the model laid out in Disney's 'Principled BRDF', this channel changes the specular intensity for non-metal materials, across a range exhibited by all real-world materials with exception to diamond. 

IoR to Specularity:

f0 = (1-IoR)^2 / (1+IoR)^2

specularity = f0 / 0.08



spec stuff, plus a chart of common values.

The material reference graphs document contains common values for what should be used here for realistic values. If however, you don't want to worry about tinkering with it, you can almost always safely set this to middle-grey, which is a common value used in many PBR implementations.