System Integration Testing is just testing in an environment which may be the first place where code from multiple projects may come in contact with each other. Usually such testing would involve the same types of things you'd test in a dev environment, except you may have access to other things that may not be present in a dev environment: APIs, dependencies, databases, etc.
SIT is more important for understanding what may be going on under the surface in your environment, than in what makes testing different. It's still testing.
