This read-only enumerated integer reports which ATM User-Network Interface version the bus is currently using: unknown(1), uni30(2), uni31(3), or uni40(4). An administrator checks this to confirm the ATM bus negotiated the expected UNI signaling version with the attached ATM switch, since a mismatch or unexpected downgrade can limit available signaling features. For example, if an admin expects UNI 4.0 signaling for newer QoS features but this object reads uni30(2), they know the connection negotiated down to an older signaling version and investigate the switch-side configuration.