Informally, this attribute can take on any reasonable abbreviation for a set of units that might be understandable by the intended audience, in either set of units (English or ISO). For greater re-usability, it is recommended that the set of measurements listed in the AIAA Flight Dynamics Model Exchange Standard [AIAA11] (of which this document is a part) be used. The Standard recommends how to encode powers of units (f_s2 for ft/sec^2, for example).