IBM 3270 Manual page 21

What is on Your Screen?
One window may fill the entire screen. When
one of your windows fills screen, it doesn't look
like a window. You can't see the other windows,
and you can use only one at a time. You can
imagine that they are stacked behind one
another like sheets of paper.
You may have several small windows on your
screen at the same time. You may see their
borders and their names or you may not. Some
of the windows may overlap others.
You can use a window only when it is the
"active" window. If your window fills the entire
screen it is active. If there is more than one
window on your screen, the active window has
the double border. It also has the cursor inside
it. Whatever you do affects only the active
window. If you want to use another window, you
must make it active.
2-2
Book 3 - Using the Control Program