Marking
Also known as: Token marking
A marking of a Petri net is an assignment of a nonnegative token count to every place. It is the global state of the net.
Firing a transition replaces the current marking with a new one: subtract tokens from input places, add tokens to output places. The initial marking $M_0$ is where analysis starts.
Reachability is the question: which markings can appear after a finite firing sequence from $M_0$? Safety and deadlock properties are statements about that set.