The extended system context describes the system interfaces and the detailed connection to the system actors and to the internal parts of the actors. In the previous post How to model a simple system context with SysML I’ve written about system context in general and the simple edition of system context that is simply spoken just a list of external systems and human actors who interacts with the system under development.
The extended system context adds information about the interfaces at the system boundary and more details about the system actors. While the simple system context is an easy-to-read-for-everyone diagram, the extended system context diagram is more specific for the engineers. Note that you need blocks as actors to specifiy internals or interfaces of the actors. See Death of the Actor for details.
The diagram is an internal block diagram (ibd). To create such a diagram you need a special system context block. It is not part of SysML, but defined as a stereotype in the SYSMOD profile. The system context block owns the system and all its actors. The diagram above is an ibd of the system context block. The diagram below shows the definition of the block:
In practice I prefer to use both system contexts. The simple system context is easy to create, a good starting point and perfect for the communication with the stakeholders. The extended system context is necessary for the definition of the system interfaces and the integration of the system in its environment.