Multi-system integration refers to related integrations that affect more than 2 systems. This either involves processes being controlled across more than 2 systems. Or that data is routed over more than 2 systems.
To implement a multi system integration you need nothing more than to set up multiple HubEngine plans. For example one from system A to system B and one from system B to system C.
The number of chained systems is not technically limited.
A schematic representation of a multi system integration you can find here.