IBM 3270 Manual page 29

It lists your windows and shows which
window is active. The window shown in the
box is the active window. For example, in
this list, window A is active.
3. If you want to remove the list to see the right
side of the screen, press the PF1 key again.
4. Press the WSCtrl key to go back to using the
active window.
2-10 Book 3 - Using the Control Program