From Vanguard Wiki
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.
 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.