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 folder of an installed instance. This is at the following location be default:
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.