When defining an attribute of type INTERREF (object reference), you have to enter both the name of the model type and the name of the class in the attribute facet "AttributeInterRefDomain".
In the current example, if you want to use the classes E and Y in more than one model type, you have to add all the model types in the facet:
1REFDOMAIN max:999
2OBJREF
3 mt:"Sample"
4 c:"Y"
5 max:999
6
7OBJREF
8 mt:"Another Modeltype"
9 c:"Y"
10 max:999