We are dealing with system of interacting processors that have to solve a common problem. There can be different degrees of interacting processors decentralization. System become mode complex with each degree.

P0 - one processor

One processor carries out everything in a sequence. We cannot talk about parallel processing here

P1 - tightly coupled processors

Two or more tightly coupled processors (that are of the same type) on one bus.

P2 - tightly coupled processors of different type

Two or more specialized processors. Typically computer has main processor and one or more coprocessors (mathematical coprocessor for example). This processor interaction type is the same as previous from connection point of view. However processes have to distinguish between different type of processors.

P3 - tightly coupled controllers

Two or more tightly connected specialized controllers.
Computer system typically has central processor and controllers for various modules that can carry out particular operations. Start / Stop system where master system starts and ends an operation while controller can carry out jobs that are usually slow in mean time. 

P4 - loosely coupled controllers

More complex P3 variety with autonomous controllers that are linked with I/O channels. 

P5 - loosely coupled autonomous computers

Two or more loosely coupled computer systems. Two computers interacting in handshake protocol for example.
Interaction coordination and job execution coordination are the main problems here. Job execution coordination can be taken care for serial, simultaneous or in combination of both modes in more autonomous computers.

P6 - network coupled autonomous computer systems

Two or more network coupled computer systems. Essence here is modified data channel that connects computers with each other. We don't have control lines here (as we have in P5) and quite more complicated data flow control has to be assured that the one in P5 is.




From P0 to P4 majority of problems can be solved within the scope of one computer with good architecture and control system (operation system) choice. For P5 and P6 processing decentralization types processing is divided between various standalone computers (with autonomous operating systems and applications).