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See boldface. How would a reader know when fresh content has been added?
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AFAIK an edit triggers a post to add up on the watchlist again, but it does not bump the thread up in the subboard view. Its reasonable to assume that regular readers either use a bookmark and can just check or the watchlist.
An edit does not make a thread show up on your watchlist. If you are viewing a sub then an edited post that you previously read will now show up as unread so unless you are specifically looking for new content on that thread then a reader will not see it.
Edit: I tried testing this using a not-so-secret alt and my old account sales thread and I was not able to get my thread to show up as being unread after editing the OP as well as after editing *my* last reply to my thread.