What is a “labeling system”?
A labeling system specifies the dimensions used for labeling, the scale of allowable values on each dimension, and a description of the criteria used in assigning values. For example, the MPAA rates movies in the USA based on a single dimension with allowable values G, PG, PG-13, R, and NC-17. Each labeling system (“schema”) is identified by a valid URL. This enables several services to use the same labeling system and refer to it by its identifier. The URL naming a labeling system can be accessed to obtain a human-readable description of the labeling system. The format of that description is not specified.
