"Modulus Of Impedance} is the absolute value of the quantity $\textit{impedance}$. Apparent impedance is defined more generally as
the quotient of rms voltage and rms electric current; it is often denoted by $Z$.
Applicable units are those of quantitykind:ModulusOfImpedance
http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=31891
http://en.wikipedia.org/wiki/Electrical_impedance
http://en.wikipedia.org/wiki/Absolute_value
$Z = \left | \underline{Z} \right |$, where $\underline{Z}$ is impedance.