It is an assessment of racehorses which was issued by the International Federation of Horseracing Authorities (IFHA) in January 2008.
It is judged that these weights would equalize the abilities of the horses if carried in a theoretical handicap race.
The IFHA publishes this information when the lower rating represents the overall top performance in a particular category.
They also show the top performers in various subdivisions of each group, which are defined by the distances of races, and the surfaces on which they are run.
The IFHA recognizes five distance categories — Sprint, Mile, Intermediate, Long and Extended — identified by the acronym "SMILE".