Distributors are not mentioned during the actual setup of test equipment, but do help define the next terminology -- cabling subsystems.
Cabling Subsystems define what type of link you are working with; i.e. backbone or horizontal. Cabling subsystems ARE mentioned during the setup of test equipment, so it is important to understand which is which.
CS3 is a type of backbone that only occurs when Distributor C (MC or MDF) is connected to a Distributor B (IC or IDF).
In this example, there is one CS3 backbone link, shown here as the blue dotted line.