What it involves
Systems integration is the work of bringing electrical, electronic and software subsystems — often from different suppliers, teams or disciplines — together into a single system that actually behaves as one. That means defining interfaces precisely through interface control documents, planning the integration sequence, and coordinating across whatever organisational boundaries sit between the subsystems, rather than assuming they'll simply fit together at the end.
Why it matters
Interfaces are where complex programmes actually fail. Individual subsystems can each pass their own tests perfectly and still fail to work together, because no one owned the boundary between them. Interface problems found during integration or commissioning are among the most expensive and schedule-damaging a programme can hit — the fix is almost always to look for them deliberately, on paper, long before hardware or code from different suppliers first meets in the same room.
What you get
Interface control documents defining every boundary precisely, an integration plan sequenced to surface risk early, independent coordination across supplier and discipline boundaries, and integration test evidence that gives you confidence before commissioning rather than during it.