This sets whether this ATM port operates as a User-Network Interface (UNI) facing a customer device, or a Network-Node Interface (NNI) facing another switch in the carrier network. This choice changes the cell header format and signaling expected on this specific port; NNI uses the full 12-bit VPI field while UNI reserves some header bits differently, so a mismatch here against what the connected equipment expects will misinterpret cell headers. A trunk connecting this switch to a peer ATM switch would be configured as nni on this port, while a port serving an end customer's ATM CPE would be left as uni.