Terminals like xterm and rvxt allows you to single left
click anywhere inside the terminal window to mark the
start of a selection buffer. Any following rightclick
will mark from the last left click to the right click.
In aterm (0.4.2) you are required to actually select
something with the left button before the left-click is
considered a start for a buffer marked with left-right.
If you don't the buffer start is considered to be at
the top left corner of the window.
I find this behaviour to be abit more inconvienient
than the behaviour of the other terminals mentioned.