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Diplomacy Parley encounters are divided into five types of conversation. The type of conversation effects which types of Expression you will be able to use as fuel during your parley.
[edit] Conversation Types
When preparing to parley, you must first check to see what type of conversation it will be (by hailing the NPC), and then construct a Strategy to overcome any restrictions on the types of Expression you can use.
Once you have your Strategy set, you can begin the Conversation which will enter you into Parley.