The task model is used to create high level system or architectural diagrams of a software system or lower level task diagrams showing threads of execution.
System Diagram
Architectural diagrams illustrate the physical design of complex systems using processors, devices and interfaces. Client-server or other distributed systems can be shown with predefined or user defined icons like computers, printers or communication systems.
Task Diagram
Task diagrams show detailed interactions between independent threads of execution (tasks or interrupt service routines, ISRs).
These interactions use operating system services including queues, semaphores, mailboxes, event flags and input/output ports.