On principle shader, roughness is the inverse of glossy. Specular is a range from 0 to 8% of IOR(Index Of Refraction) the fresnel. I believe it only applies to dielectric(metalness less than 0.9). I don’t remember if it’s Disney or Pixar that established this use of specular, but either way Blender adopted it. It is not the same as specular PBR workflow. Blender’s principal shaders are metalness workflow. Which breaks down to metal(dielectric), roughness(glossiness), and base color(albedo preferably). If you want to convert IOR to specular the formula is ((IOR-1)/(IOR+1))²/0.08
