  The dynamic critical exponent $z$ is determined from numerical simulations
for the three-dimensional (3D) lattice Coulomb gas (LCG) and the 3D XY models
with relaxational dynamics. It is suggested that the dynamics is characterized
by two distinct dynamic critical indices $z_0$ and $z$ related to the
divergence of the relaxation time $\tau$ by $\tau\propto \xi^{z_0}$ and
$\tau\propto k^{-z}$, where $\xi$ is the correlation length and $k$ the
wavevector. The values determined are $z_0\approx 1.5$ and $z\approx 1$ for the
3D LCG and $z_0\approx 1.5$ and $z\approx 2$ for the 3D XY model. It is argued
that the nonlinear $IV$ exponent relates to $z_0$, whereas the usual
Hohenberg-Halperin classification relates to $z$. Possible implications for the
interpretation of experiments are pointed out. Comparisons with other existing
results are discussed.
