This identifies which half of a redundant pair this card currently is, actively carrying traffic or sitting in standby ready to take over. The MIB's own exception note matters operationally: when this value is used as the index into cardAdminType, cardAdminCapability, and cardAdminStatus, the values 1 and 2 stop meaning active and standby and instead just mean the card in the lower-numbered slot versus the card in the higher-numbered slot, so a script that assumes 1 always means currently active can misreport things. For example, a monitoring script that assumes cardRedundState equal to 1 always means currently active will misreport which card is really carrying traffic if it's actually being used as a slot-position index rather than a live status read.