I was reading in the article A tutorial on partially observable Markov decision processes (p. 120), by Michael L. Littman, that $\sum_{z \in Z}O(a, s',z) =1$, where $a$ is the action, $s'$ the next possible state and $z$ a certain/specific observation.

How come that the observation function $O(a, s', z)$ adds up to $1$ in POMDP?