An exchangeable phrase list feature are serves as single feature to the LUIS underlying training algorithm. It is used as a lexicon lookup feature where its value is 1 if the lexicon contains a given word or 0 if it doesn’t. A non-exchangeable phrase list feature has all the phrases in the list serve as separate features to the underlying training algorithm. So, if you your phrase list feature contains 5 phrases, they will be mapped to 5 separate features. You can think of the non-exchangeable phrase list feature as an additional bag of words that you are willing to add to LUIS existing vocabulary features.