The distinguished field is used to expose the value of a node of a message instance to BizTalk Server system. However, if there is a record that has a parent node that has multiple instances, then that record won’t be able to be highlighted as distinguished. An error message of “The node can occur potentially multiple times in the instance. Only nodes which are guaranteed to be unique can be promoted.“ will appear.
This constraint was purposely designed to remove any ambiguity in accessing the data. However, despite that, should the developer still desire to use the node, then the record’s XPath query can still be used. The article Using XPaths in Message Assignments describes how XPath queries can be leveraged in BizTalk Server.