USIM Application Toolkit (USAT) is the equivalent of STK for 3G networks. USAT enables the USIM to initiate actions which can be used for various value-added services delivered over mobile devices.
USAT takes advantage of the multiapplication environment of 3G devices by not activating until a specific application has been selected, unlike STK which is activated at startup. Some functions are card related rather than application related.
USAT is defined in standard 3GPP 31.111 for 3G.
More information
For more information see STK, the equivalent of SAT for 2G.
References
USIM Application Toolkit Wikipedia(Text) CC BY-SA