This is the distance between the plates of glass in your unit. Usually between 6 and 20mm. You get better energy efficiency (avoid heat loss) with a spacer bar width over 10mm but this may be impossible depending on the overall width available for the particular window frame. Low emission glass can increase energy efficiency still further and compensate where narrower panels have to be used.
The spacer bar colour option is to enable you to match the colour in the room or existing panels.
Argon-filled units increase the thermal efficiency of the double glazing. Keeping more heat in the room as the gas insulates better than air.
