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 No Dimensions) or blank if the signal is a dimensionless quantity or flag.

Informally, this attribute can take on any reasonable abbreviation for a set of units that might be understandable by the intended audience, in any set of units (English or ISO). For greater re-usability, it is recommended that the set of measurements listed in the AIAA Standard for Flight Simulation Models, of which this document is a part, which also suggests how to encode superscripted powers of units (ft_s-1 for ft/sec, for example).