Added a note about some common concepts that are not usually software…

… systems.

abstractions/01-software-system.md

@@ -19,4 +19,7 @@ something a single software development team is building, owns, has responsibili
 implementation details of. Perhaps the code for that software system resides in a single source code repository,
 and anybody on the team is entitled to modify it. In many cases, the boundary of a software system will correspond to
 the boundary of a single team. It may also be the case that everything inside the boundary of a software system is
-deployed at the same time.
+deployed at the same time.
+
+Things that are not usually software systems in the C4 model include product domains, bounded contexts,
+business capabilities, feature teams, tribes, or squads.