How to retrieve the "Used in" information of a requirement artifact with client extension?

In the documentation for RM client extensions (https://jazz.net/wiki/bin/view/Main/RMExtensionsAPI502) I found under  "Well known attributes and properties"

CONTAINING_MODULE  = The Used In property of an artifact that refers to a containing module

I expected to get the references as displayed in the "Where Used" section of the artifact.

the code looks like this

RM.Data.getAttributes(refs,  [RM.Data.Attributes.IDENTIFIER, RM.Data.Attributes.CONTAINING_MODULE], function(opResult) {
   if (opResult.code === RM.OperationResult.OPERATION_OK) {
        opResult.data.forEach(function(item){
        var id = item.values[RM.Data.Attributes.IDENTIFIER];
        var used = item.values[RM.Data.Attributes.CONTAINING_MODULE];
        });
});

The "id" is ok. For "used" I always get the value "undefined".

What is wrong?