TERR functions are a subset of the built-in expression functions in Spotfire that invoke the TERR engine for advanced statistical analysis. You can find them in the Function list of the Custom Expressions or Insert Calculated Column dialog box, under the category Statistical functions. TERR functions have the prefix TERR_ or TERRAggregation_ followed by the Spotfire data type of the output. For example, TERR_Real, TERRAggregation_String, etc. You can also register your own TERR scripts as expression functions and assign them to a custom category. References: Built-In TERR Expression Functions in Spotfire, Registering the TERR Script as an Expression Function