The DBX cards in all systems are wired such that if you could unplug the DBX cards with their port wiring intact and lay them out on the ground untangled, the DBX cards and the wiring connections would form a giant circle.
If all the links in a DBX system are up and running, it is possible for a DBX card to send a message out its PORT A, and receive the same message back on its PORT B after the message had passed through every other DBX card in the system (assuming that every card that received a message on one port then transmitted the message on its other port).
The purpose of this configuration is to allow any DBX card to have at least two paths to every other DBX card using coax only. A card can transmit out either of its ports to reach another card as long as all links are up. If a single link goes down (a link being a pair of coax cables for TX and RX), every card can still reach every other card using coax only by going out either its PORT A or its PORT B since a broken circle still has all the points connected by a single line.