Transition

Transitions represent process steps. A transition becomes active when all its input places are tagged and all its output places empty. Input places are those that are connected to the transition by an edge pointing from the place to the transition, whereas the edge between an output place and a transition points just the opposite way. When the transition is active, it can fire.

Upon firing, the transition

1. removes one token from each input place
2. executes the process step
3. and places a new token in each output place