Width is a ratio relative to the center of the family. The regular or default face has, by definition, a width of 1.0. The recommended measurement method uses the total length of the Shannon Text in mixed-case setting. In A Time When Whom Ever Told The Problem To An Unexpected. These words represent the frequency of letters in typical English.
Weight is a ratio relative to the center of the family.
For Roman faces, weight is typically measured by the thickness of vertical strokes. The regular or default face has, by definition, a weight of 1.0. Apple does not recommend any particular method of measuring weight. Registering your descriptor A manufacturer may want to include other pieces of metric information with its fonts.