This read-write setting configures whether cells transmitted on this physical line use the ATM UNI or ATM NNI cell header format, or some other format. Getting this right matters because UNI and NNI headers allocate VPI bits differently, so a mismatch with the attached device would break cell interpretation on the line. An admin connecting this line to another switch as a network trunk would set atmLineInterfaceFormat to atmNni(3) rather than the UNI setting used for customer-facing access lines.