Flat Mode is the default mode, showing only the sequence of posts as they were entered. Threaded mode typically (but not always) shows the first post of a thread, followed by a listing of all of the thread's posts as clickable links, and indented so as to show what post each subsequent post was answering. E.g., if you click "Threaded" on this thread, it will show your complete post. Below that is the list with yours at the top, TW's below and indented from yours to show that he replied to yours, and mine below and indented from TW's, because I used quick reply, which assumes a reply to his.<pre>Your post
TW's post
My post</pre>If I used the Post Reply button right above Quick Reply, it would be aligned directly below TW's, because it always assumes a reply to the opening post in the thread.<pre>Your post
TW's post
DD's post</pre>Ditto using the Reply link in your post in this case, although in a more complex thread, the Reply link would align it one indent to the right of the post replied to, no matter where it was.<pre>Your post
TW's post
DD's post
Your 2nd post
X1's post
X2's post
X3's post
X4's post
DD's reply to your 2nd</pre>It allows immediately seeing who replied to whom, but it's less convenient in terms of reading the thread.
UBB 101 is dismissed.