This gives the height, as a layout dimension string, of a single slot cell in this array's rendered diagram. UCS Manager uses this together with width and the offset fields purely to draw an accurate, proportionally scaled front or rear panel picture of the chassis in its GUI, not to affect how the hardware itself operates. A typical value is a small dimension string such as '20', sized so that, say, 24 drive-bay cells fit proportionally across the rendered chassis diagram.