  We show that the GENERIC model for relativistic heat conduction is a
multifluid of Carter. This allows one to compute the multifluid constitutive
relations directly from the GENERIC formalism. As a quick application, we prove
that, in the limit of infinite heat conductivity, GENERIC heat conduction
reduces to the relativistic two-fluid model for superfluidity. This surprising
``crossover'' is a consequence of relativistic causality: If diffusion happens
too fast, all the diffusing charge cumulates on the surface of the lightcone,
and it eventually travels at the speed of light like a wave. Our analysis is
non-perturbative, and it is carried out in the fully non-linear regime.
