Conflict is a situation in Petri net modelling such that two or more transitions are enabled but firing of any one of them leads to disabling of others.
Figure b: Conflict in a Petri Net
Now transitions t1, t2, t3 are in conflict. The situation usually arises while a machine has to choose from various part types or a part has to choose in between different machine types. Such conflicts are solved employing a purely a non-deterministic method, or in a probabilistic way, by allocating appropriate probabilities to the conflicting transition.