  We use 1697 clusters of galaxies from the Illustris TNG300-1 simulation (mass
$M_{200c}>10^{14}$M$_\odot$ and redshift range $0.01\leq z \leq 1.04$) to
explore the physics of the cluster infall region. We use the average radial
velocity profile derived from simulated galaxies, ${\rm v_{rad}}(r)$, and the
average velocity dispersion of galaxies at each redshift, ${\rm \sigma_v}(r)$,
to explore cluster-centric dynamical radii that characterize the cluster infall
region. We revisit the turnaround radius, the limiting outer radius of the
infall region, and the radius where the infall velocity has a well-defined
minimum. We also explore two new characteristic radii: (i) the point of
inflection of ${\rm v_{rad}}(r)$ that lies within the velocity minimum, and
(ii) the smallest radius where ${\rm \sigma_v}(r)$ = $|{\rm v_{rad}}(r)|$.
These two, nearly coincident, radii mark the inner boundary of the infall
region where radial infall ceases to dominate the cluster dynamics. Both of
these galaxy velocity based radii lie within $1\sigma$ of the observable
splashback radius. The minimum in the logarithmic slope of the galaxy number
density is an observable proxy for the apocentric radius of the most recently
accreted galaxies, the physical splashback radius. The two new dynamically
derived radii relate the splashback radius to the inner boundary of the cluster
infall region.
