I think it is pretty straightforward. How many phases do you have? If you have more than 2 then it is possible to have an initial condition that has a volume fraction more than 1. For example if you have three phases, then you can patch VF2 as 1.0 but have already said that VF1 is 0.5 at another part of the initialization then you have a total Volume Fraction over 1. You need to set the VF1 to be 0 to insure that you have no parts over your domain as over 1.
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