Smart Sharpen: Highlight/Shadow Radius "controls area in which PS looks"—isnt PS lokng at whole img?

hi,

Im a bit confused to what "radius" in the highlight and shadow section of smart sharpen is doing. The highlight and shadow section is dampening or undoing the main section of smart sharpen, however the bit I struggle with is what some websites describe the function of radius in the shadow and highlight is.."Radius value controls the width of the area around each pixel that Photoshop looks at to determine if the pixel fits within the specified tonal range" — Isn't PS looking throughout the entire image to see which pixels are within the tonal range anyway? Can someone in simple terms explain this one to me or am I better off just to ignore the shadow and highlight section and just mask away/in areas for smart sharpen to affect? thanks