This paper deals with the structure theory of Petri nets. We define the class of P/T systems namely K-systems for which the equivalence between controlled-siphon property (cs property), deadlock freeness, and liveness holds. Using the new structural notions of ordered transitions and root places, we revisit the non liveness characterization of P/T systems satisfying the cs property and we define by syntactical manner new and more expressive subclasses of K-systems where the interplay between conflict and synchronization is relaxed. For further information, please visit this web site.