ThreadMode is for arguments between two or more people; DocumentMode captures the consensus resulting from a debate in ThreadMode.  DocumentMode texts are usually unsigned and written in the third person; they may include a contributor list at the bottom.

EssayMode superficially resembles DocumentMode, but represents one individual's point of view.  EssayMode can be recognized by its use of the first person and by the presence of a signature.
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So EssayMode is like a conversation, a ThreadMode thing but with only one person? -- DaveHarris