A Process Module is a set of process elements that can be added to other processes. A process module can also contain other process modules, so process modules can be used to build a process hierarchy or process architecture.
Typical process modules are Requirements Engineering, Design, or Project Management. As process modules can be used to separate common from specific processes, a “Design” process module could be used to contain the common elements in a “Engine Design” and “Transmission Design” process module.
The only difference between a process and a process module is that a process module can be added to other processes or process modules, whereas a process can not be added anywhere else.