CON: stands for Console-Handler. This is a program which handles
keypresses and textual outputs on its window similar to a file.

You have to pass several things when opening a CON: window:
CON:leftpos/upperpos/width/height/title
leftpos,upperpos are the position of the left top edge in pixels.
  Both need to be 1 or more.
width,height are the window's dimensions in pixels.
title is a string

With OS2.0+ you can pass some more things and you can drop some things.
So "CON:" alone is valid and opens with the default settings.
To get a close gadget you have to append "/CLOSE", eg.
    CON:1/1/600/100/Great/CLOSE
"/SCREEN screenname" opens on a public screen
"/WAIT" stays till you click the close gadget or press CTRL-
"/ALT" gives an alternative size with the zoom gadget
... there are some more but I do not know much of them