Systems are build from Sub-Systems arranged in Networks of Sibling Systems. So when you're interested in some Systems you may look sideways to the sibling systems, up the hierarchy to the Super-Systems and down the hierarachy to the Sub-Systems. When you're responsible for the development of some systems you're interested in the integration of sub-systems to create your system, looking good toward the sub-system and collaborate with the sibling systems without being hurt and without hurting others.
But there is one place where there is nothing to look up for and that's the place of the ultimate user of the system. The user is interested in getting value from her technologies, to look good toward her peers, bosses and other people. The integration here is social not technological like in lower levels of integration and it's always networked not hierarchical.
Three levels of integration thus may be discerned:
1. Product integration of the technological artefacts
2. System integration of the products and their users
3. Business/Social/Operational integration of the technology-wielding users in their social networks
Every level has its own pecularities and its own chalenges - but almost all SE literature is concerned with product-level integration leaving the two other levels to the users - no so fair!