Currently, the RACS in the ISX Designer supports only the SX 42U and SX 42U Wide Rack, namely AR3100 and AR3150. Hence, when you are trying to configure RACS with an AR3140, which is a Network Rack SX 42 Wide, it is not adding the Rack Air containment assembly. The Rack Air Containment System does not support the AR3140.
However, as per your concern, we will forward this feedback to the concerned team and see if the network racks can be included for RACS in the ISX Designer. We will update you with more relevant information.