The correct expression to use for the results filtering expression to filter by step type is : Result.TS.StepType
For instance, you to create a report which only shows the step type Multiple Numeric Limit Test which have failed, you can use the expression : Result.TS.StepType == "NI_MultipleNumericLimitTest" && Result.Status == "failed"

Note: The result filtering expression is not available when using the XML format, or when using the ATML format with TestStand 2010 SP1 or previous. In these cases, you can filter results through modifying the report style sheet.