Screening reduces the 'saturation' of the color. For inkjet plotters/printers, this is fewer 'dots per unit area', so the color is 'lighter'.
We use this to get 'faint' colors. For example, color 3 with a 10% screening gets you a faint 'lime green' useful for solid hatching/highlighting an area inside a REVCLOUD.
The hatch will still completely cover whatever may be 'behind' it, so make sure that the highlight hatching layer(s) are at the 'back' of the objects in your drawing.