Out-of-band simply refers to communications which occur outside of a previously established communications method or channel. In the case of cisco prime and being part of the config archive I suspect it is refering to out-of-band management ports which are now part of all new cisco switches (2960S, 3750X, Nexus 7K and 5K, etc). The out-of-band port is designed to continue to provide console access even in the even of a primary network subsystem failure. Hence in a properly designed network there would exist a entirely seperate out-of-band network which relies on none of the hardware or software forming part of the primary production network.
In the case of being part of the config archive I would then conclude that the out of band column simply relates to all the config for out of band managment and the associated port, so all the config for any fa0 Layer 3 OOB Management port that exists on a switch/router would appear here, as might any vrf config that seperates the OOB port from the rest of the network.
