Glass surfaces are identified by number, starting with the exterior surface, which is always Surface Number 1. Each pane of glass has two surfaces, so there are always an even number of glass surfaces in any glazing unit.
It is important to correctly identify the glass surface number because decorative or energy efficient coatings are applied to the glass based on the surface number. In addition, the coating can have a different appearance or may function better on certain surfaces. For double-pane insulated glass units, coatings are usually added to surface #2 and surface #3 so that the coating is "inside" the unit, which protects if from damage.