What is the name for this fare construction?

I asked matrix.itasoftware.com to find me a multicity itinerary with three components:

(1) LON->CAK,CLE
(2) CLE->STL
(3) STL->LON

It gave me the following itinerary:

Day 1: LHR-ORD-CAK
Day 3: CLE-ORD-STL
Day 5: STL-ORD-IAD-LHR

It gave me fare construction:
Fare 1: LON->CAK
Fare 2: CLE->LON
Fare 3: CHI->STL
Fare 4: STL->CHI

How does that work? It looks like this fare engine is starting the return segment as "CLE-ORD-IAD-LHR", and then adding on an "ORD-STL // STL-ORD" segment inside there. What is this construction called?