PHOEBE 2.0.x defined the size of a star by its polar radius, rpole. In addition, there was a constraint between rpole and the equipotential, pot, which could be reversed by the user so that pot defines the size. When using different values for distortion_method (roche, rotstar, sphere, etc), the polar radius was conserved.
However, with the introduction of misalignment in PHOEBE 2.1, this way of parameterizing the stars became insufficient. Starting in PHOEBE 2.1, the sizes of stars are now parametrized by the equivalent radius, requiv. When using different values for distortion_method, it is the value of requiv that is now conserved. Thus, in general, polar radius is no longer conserved.
For contact binary systems, there is an requiv parameter for each star (or each "half" of the contact system), as well as a constrained parameter for the potential (pot) and fillout-factor (fillout_factor) for the envelope as a whole. As contact binary systems must be aligned, circular and synchronous, and only support distortion_method='roche', this mapping is still possible and provides significant conveniences. By default, the requiv of the primary star is adjustable by the user, but the constraints can be reversed so that any one of these four parameters can be set, with the rest being constrained.
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