A capital is the topmost part, the head, the “cap” of a column or pilaster.
These capitals have flat backs, and are meant to sit atop pilasters. A pilaster is a rectangular column with a capital and a base, which is used on a wall or façade as a decorative motif. The width dimension shown is the base width of the capital, where it meets the pilaster.
These capitals are round at the base, meant to sit atop fully round columns. Some are one piece, some are cast in halves. Price is for full capital, whether one or two pieces. The given width dimension is for the base diameter of the capital, where it meets the column shaft.