Each variable definition includes a mandatory units attribute. This attribute gives the units-of-measure for the signal represented by the variable and either 'nd' (for non-dimensional) or blank if the signal is a dimensionless quantity or flag. In addition, the use of 'fract' to indicate a fraction (0-1) or 'pct' to indicate a percentage (0-100 or more) is encouraged.

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).