  In the outer regions of a galaxy cluster, galaxies may be either falling into
the cluster for the first time, or have already passed through the cluster
centre at some point in their past. To investigate these two distinct
populations, we utilise TheThreeHundred project, a suite of 324 hydrodynamical
resimulations of galaxy clusters. In particular, we study the 'backsplash
population' of galaxies; those that have passed within $R_{200}$ of the cluster
centre at some time in their history, but are now outside of this radius. We
find that, on average, over half of all galaxies between $R_{200}$ and
$2R_{200}$ from their host at $z=0$ are backsplash galaxies, but that this
fraction is dependent on the dynamical state of a cluster, as dynamically
relaxed clusters have a greater backsplash fraction. We also find that this
population is mostly developed at recent times ($z\leq0.4$), and is dependent
on the recent history of a cluster. Finally, we show that the dynamical state
of a given cluster, and thus the fraction of backsplash galaxies in its
outskirts, can be predicted based on observational properties of the cluster.
