Consider an affine system with a polytope as state set. State trajectories are terminated when they reach a facet of the polytope and attempt to exit. The realization problem is considered based on the behavior of the system, i.e. the set of input-output trajectories on time-intervals of either finite or infinite length. The state set can be affinely reduced due to non-observability if and only if a subspace of the classical unobservable subspace, characterized using the normal vectors of the exit facets, is nontrivial. AMS Subject Classification (1991): 93B20, 93B15.