"Volume Fraction" is the volume of a constituent divided by the volume of all constituents of the mixture prior to mixing. Volume fraction is also called volume concentration in ideal solutions where the volumes of the constituents are additive (the volume of the solution is equal to the sum of the volumes of its ingredients).
Applicable units are those of quantitykind:VolumeFraction
http://en.wikipedia.org/wiki/Volume_fraction
https://cdd.iec.ch/cdd/iec61987/iec61987.nsf/ListsOfUnitsAllVersions/0112-2---62720%23UAD240
"Volume Fraction" is the volume of a constituent divided by the volume of all constituents of the mixture prior to mixing. Volume fraction is also called volume concentration in ideal solutions where the volumes of the constituents are additive (the volume of the solution is equal to the sum of the volumes of its ingredients).
$\varphi_B = \frac{x_B V_{m,B}^*}{\sum x_i V_{m,i}^*}$, where $V_{m,i}^*$ is the molar volume of the pure substances $i$ at the same temperature and pressure, $x_i$ denotes the amount-of-substance fraction of substance $i$, and $\sum$ denotes summation over all substances $i$.
http://www.iso.org/iso/catalogue_detail?csnumber=31894