Telerik Reports created in Visual Studio support events like ItemDataBinding, ItemDataBound, NeedDataSource for Data items and Error event for reports. events are trigerred when data and the designed report start being processed by the Reporting Engine.
Events are a legacy approach for modifying styles, providing data, and logging information about the processing report.
Telerik Reporting provides different declarative means that replace the need for events and provide the needed flexibility to handle most tasks: Using Expressions, Conditional Formatting, Bindings and User Functions. These tools are typically the most productive way to build declarative reports and should be favored over using events. Additionally events can't be used with Standalone Report Designer report documents, because the custom code from events is not serialized in reports.
Designed reports' structures should not be modified in events. Changes as of Telerik Reporting R3 2016, in the internal processing of reports, may affect reports using events upgraded to the latest version. For more details, please check Changes on items in report events are not applied.