The number of window units is the total quantity of separately installed sections of window framing, regardless of the number of panes joined by the framing sections. A window unit can include one window pane or several. Multiple window units can fill a single window aperture.

Window units do not include entrance doorways, balcony doorways, or skylights. They must be installed in the perimeter walls of a building, or in vertical or essentially vertical sections of the roof, such as dormers.

The three window counts describe the same windows differently and rarely match. One opening in the wall is one aperture; the frame set into it is one unit; the sheets of glass in that frame are several panes. A traditional sash window is one aperture, one unit and a dozen panes.

A source that says a building has a number of windows has not said which of the three it counted, so a figure taken from prose belongs in whichever field its wording actually fits.

Insulated glazing does not multiply the pane count. A double-glazed sheet is one pane, because the field counts divisions of the window and not layers of glass.