The transom is the center section. Some boats have transoms that run all the way from one side to the other, some do not.That center piece is doubled 3/4 ply, what you are calling the outboard sections are merely backers for the swim platform which on my boat is full length. Those pieces I chose not to replace because they are not rotted, don't tie into any other piece and as far as I can tell the only reason they are there is to provide support for the platform thru bolts.