The Dundas.BI.WebApi assembly is a .NET assembly that contains the classes that are used when communicating with the Dundas BI REST Services.
As a consumer of this assembly the only parts that are relevent are the items under the Dundas.BI.WebApi.Models namespace. These the are the types that can be serialized as JSON and passed to and from the Dundas BI REST Services.
The Dundas.BI.WebApi 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.WebApi.dll
Alternatively this assembly can be added by referencing it as a NuGet package.
This assembly also references Dundas.BI.Core.dll, and Newtonsoft.Json.dll, which can be found in the same sdk bin folder as above.