A process definition represents a formal specification of a business process and is based on a directed graph. The graph is composed of nodes and transitions. Every node in the graph is of a specific type. The type of the node defines the runtime behaviour. A process definition has exactly one start state.

A token is one path of execution. A token is the runtime concept that maintains a pointer to a node in the graph.

A process instance is one execution of a process definition. When a process instance is created, a token is created for the main path of execution. This token is called the root token of the process instance and it is positioned in the start state of the process definition.

A signal instructs a token to continue graph execution. When receiving an unnamed signal, the token will leave its current node over the default leaving transition. When a transition-name is specified in the signal, the token will leave its node over the specified transition. A signal given to the process instance is delegated to the root token.

After the token has entered a node, the node is executed. Nodes themselves are responsible for the continuation of the graph execution. Continuation of graph execution is done by making the token leave the node. Each node type can implement a different behaviour for the continuation of the graph execution. A node that does not propagate execution will behave as a state.

Actions are pieces of java code that are executed upon events in the process execution. The graph is an important instrument in the communication about software requirements. But the graph is just one view (projection) of the software being produced. It hides many technical details. Actions are a mechanism to add technical details outside of the graphical representation. Once the graph is put in place, it can be decorated with actions. The main event types are entering a node, leaving a node and taking a transition.