PCB industry in many processes have a window that is only a technical term in this industry.
Line window: that the production line layer window is the need to retain a portion of the positive film.
If the window is on the negative film is the need to remove the part.
Solder window: The window film the way, the need for welding or heat to the PAD exposed outside the green oil to achieve the desired customers.
There are text windows and the like.
Simply put, this means that a window of copper etched away, had a piece of copper is in the middle of an area no copper,
It is equivalent to the copper layer to open a window.