This read-write enumerated value configures the ATM interface signaling type of a port as UNI, NNI, STI, or the special vpTrkUni mode, with the description specifying that STI is supported only on IMATM cards and vpTrkUni is used specifically on IMATM when virtual trunks pass through it using a UNI cell format while still carrying ForeSight rate information in the lower 16 bits. Admins configure this to match how the port is being used in the network topology - user-facing (UNI), network-to-network (NNI), or the IMATM-specific trunk-passthrough modes. For example, an admin configuring an IMATM port that's carrying a virtual trunk needing ForeSight congestion feedback preserved end-to-end would set ausmPortType to vpTrkUni rather than a standard UNI or NNI setting.