Sometimes, we need to develop complex reports, for example, reports that involve more than one related entity or where we need to use expressions (we can use expressions in reports to define the content of SSRS controls) to validate the value of a field. These types of reports are not possible to develop using the Report Wizard, so we use Visual Studio to develop them. In order to write custom reports in Visual Studio, we need to make sure SQL Data Tools is installed with the Report Authoring Extension for Dynamics 365 CE. The Report Authoring Extension is needed for writing FetchXML reports.
You can find more information on expressions here: https://docs.microsoft.com/en-us/sql/reporting-services/report-design/expression-examples-report-builder-and-ssrs?view=sql-server-2017 ...
