An external reference attribute is a single or an array of multiple references in the form of a URI and a reference name. Below are examples of how to handle them using the API. 

Example

IswItem i = SWConnection.Instance.Broker.GetItem(SWHandleUtility.ToHandle("x040000000000002D"));
IswAttributeType externalReferenceType = SWConnection.Instance.Broker.AttributeType("EXTR");
IswAttribute attr = i.GetOrMakeAttributeOfType(externalReferenceType);

// Read value from External reference attribute
string value = attr.ValueAsXML;

// Write value to External reference attribute
attr.ValueAsXML = "<ExtRef uri=\"C:\\Users\\Fredrik\\Desktop\\Mamma mia.jpg\">Mamma mia.jpg</ExtRef>";