What is the effect of "Notes," in which O'Brien explains the story behind "speaking of Courage"? (Which parts are true and which parts are the...

"Notes" is a complement and a sequel to "Speaking of Courage."  It is the third and final account of Kiowa's death, and the second post-war account of it.  It is the only one of the three to be written in first person, from Tim's perspective.  It is also an example of metafiction, fiction about fiction.

As you know, The Things They Carried is more about storytelling than war.  It is an exercise in memory and bringing the dead back to life, in this case Bowker.  These are love stories, not war stories.  To tell his readers at the end of "Speaking of Courage" that Bowker killed himself would have drained the story of it metaphorical meaning: it would have been a sensationalistic and dehumanizing ending.  Instead, he presents it as its own epilogue, which shows how Tim has worked and re-worked the story, like his guilt.  His readers can tell that he is still not over Kiowa or Bowker's death, and his stories are an attempt to reconcile the past and the present.

All the parts are true, even though they may not literally have happened.  O'Brien, of course, imbues the facts with fictional elements to "heat up the truth" and give the story continuity.  But most of the accounts in the story are taken from the 17 page letter that Bowker wrote O'Brien.  O'Brien may have condensed, added, or omitted characters, plot elements, and settings as needed, but as he says in "How to Tell a True War Story," a true war story is never about war, and it is true by the simple fact that it is never finished being told.