[the definition of] how general object types and event types can be more specifically characterized in terms of subtypes.
For example: air vehicle < aircraft < aircraft category < aircraft class < aircraft type < aircraft model < aircraft series < airframe.
ICAO (2009) doc. 9883, Manual on Global Performance of the Air Navigation System
Classification schemes allow the breakdown of summary data into details, and vice-versa, they can be used to aggregate detailed (granular) performance data into summary data. (same source)
Lexicon Release 2015.2