The Dundas.BI.Web.Core assembly is a .NET assembly that contains interfaces to be used by plug-ins for the web. This allows for providing new routes and/or custom adapters.
The Dundas.BI.Web.Core assembly is located in the SDK subfolder of an installed instance. For example, this is the following location by default on Windows:
C:\Program Files\Dundas Data Visualization Inc\Dundas BI\Instances\[instance name]\sdk\bin\Dundas.BI.Web.Core.dll
Alternatively this assembly can be added by referencing it as a NuGet package.
This assembly also references Dundas.BI.Core.dll, which can be found in the same sdk bin folder as above or on NuGet.