A "known type" is a type which the DataContractSerializer knows how to serialize. DevForce uses this serializer in Silverlight applications, or in any n-tier application in which the Object Server UseDCS flag has been set. DevForce will automatically discover most known types, but in some situations you will also need to specify the known types for DevForce to use. You can do this in several ways: 1) use this marker interface, 2) decorate the type with the DiscoverableTypeAttribute, 3) decorate an existing known type with the System.Runtime.Serialization.KnownTypeAttribute, or 4) implement the IKnownTypeProvider to programmatically add known types.

The types which you need to make known to DevForce are typically those used with the EntityManager.InvokeServerMethod or the Notification Service.

Reference

IKnownType Members
IdeaBlade.EntityModel Namespace