Complex sentence: The transitive reduction and transitive closure are both uniquely defined for DAGs; in contrast, for a directed graph that is not acyclic, there can be more than one minimal subgraph with the same reachability relation.
Split into simpler sentences: The transitive reduction and transitive closure are both uniquely defined for DAGs. In contrast, for a directed graph that is not acyclic, there can be more than one minimal subgraph with the same reachability relation.