Report Properties Dialog Box, References (Report Builder)
Select References on the Report Properties dialog box to add or remove references to custom or other external assemblies and custom class instances that are used by expressions in the report definition. Custom assemblies are not supported in local mode in Report Builder. To author reports that use custom assemblies, use Report Designer in SQL Server SQL Server Data Tools (SSDT). For more information, see Comparing Report Authoring Environments in the Reporting Services documentation in on msdn.microsoft.com.