P() = Exp;

where P is the process name, and Exp is a process expression. The process expression determines the computational logic of the process. To define Exp, please refer to 3.1.1.2 Process Definitions of CSP module. There is an important constraint in this declareation is that the Exp can only contains primitive processes which are defined by drawing it as a Finite State Machine (FSM). Process referencing does not allow recursion. In other word, the process P could not call itself or use another process definition which is not a primitive process. To define a primitive process, please refer to 3.5.1.2 Primitive Process Definitions.

For example, a system Sys can be defined in such following ways:

                     System = e1 ? P1() [] e2 ? P2(); 

                     System = P3(1) || P4();

where P1(), P2(), P3(i), P4() are primitive processes defined in the Drawing part.