This read-write object indicates the cell format used on the BNM's outgoing trunk: bnmSti for the switch's internal STI framing (fixed for T3 and E3 cards), or bnmUni/bnmNni for standard ATM UNI or NNI interface framing, which BNM-155 (SONET) cards can support. An admin connecting a BNM-155 trunk to another vendor's ATM switch, rather than to another Cisco WAN switch card, would need to set this to bnmUni or bnmNni as appropriate instead of the Cisco-internal bnmSti format. Getting this format wrong would prevent the trunk from interoperating correctly with the far-end equipment.