Really what you see in Systems Integration is a network of things interacting with each other in various ways depending on the external (or internal) excitations. You see matter, energy and data flow back and forth. And frequently your may discern the parts of the network that has much more interactions inside than outside those parts - so you can (for a time being) abstract that part as being an entity on higher level of hierarchy than the nodes of the network.
When you design the system you're free to abstract parts of it, define requirements from these parts, interfaces and input-output relations. That's convenient. In the Integration you are frequiently deprived of this luxury - everything looks like the network that doesn't organize itself naturally into the neat hierarchy of the design. Most surprizes are in those situations when the neat hierarchy shows its (unexpected) network-ness.
So the Second Secret of Integration may be formulated as "Everything is a Network - Hierarchy is an Illusion".