The reference scale (RS) of VBF is related to renormalization and
factorization scales, i.e. the scales at which the strong coupling and
parton distributions are evaluated.
For the central value of the cross-section, the ren/fact scales are
simply set equal to the RS, while they are varied around it in order to
assess the theoretical uncertainities.
Actually the code allows to chose the RS among three options: the Higgs-boson mass (mh), the W-boson mass(mw) or the virtuality of the vector bosons(Q).
If either the choice "mh" or "mw" is made, this scale is set as RS for both the
structure functions. By converse, the choice "Q" set the RS of each
structure function equal to the virtuality of the attached vector boson.
In the latter case the two structure functions are evaluated with two
different reference scales.
Because the typical virtuality of the vector boson is of 20-30 GeV, well below the Higgs mass, choosing Q as RS looks more natural with respect to the Higgs mass, in particular it leads to a better perturbative behaviour for the total cross-section.
The choice "mw" is an intermediate choice between "mh" and "Q", and it can be also used to compare NNLO numbers with the NLO (QCD+EW) computation by Ciccolini, Denner and Dittmaier (arXiv:0710.4749) implemented in HAWK.