<<datatype>> A vertical distance value. For example, Upper and lower limits of an airspace, an ATS route, FL100, 600M, 300 FT, etc.. This data type also allows some special non-numerical values: GND - meaning "the Surface of the Earth"; UNL - meaning "unlimited"; FLOOR - meaning "the bottom of the airspace"; necessary to express usages for Airspace that have a non-constant lower limit; CEILING - meaning "the top of the airspace"; necessary to express usages for Airspace that have a non-constant upper limit.