Metrizable ball structures

A ball structure is a triple $(X,P,B)$, where $X$, $P$ are nonempty sets and, for any $x\in X$, $\alpha\in P$, $B(x,\alpha)$ is a subset of $X$, $x\in B(x,\alpha)$, which is called a ball of radius $\alpha$ around $x$. We characterize up to isomorphism the ball structures related to the metric spaces of different types and groups.