Windows are numbered 1 to 5  (identified as LH or RH)
Windows are heated to improve there impact resistance and to help prevent misting fogging. They are turned on at least 10min's before departure to allow them to warm up. There are two settings Low and High. Low is first used to prevent any thermal shock  to the window and allow a more gentle warming up, then during the taxi out they are placed in the high position for flight for normal operations.

Window Failure
Window failure does not compromise the integrity of the flight,  it just changes your operating procedures and limitations. I've had windows fail at altitude the only thing you do is spill your coffee, as it goes bang.

PANE                                        WINDOW                                            ACTION
Outer Pane                        All Windows                                   Normal Operations
Middle Panes                        Window 4                                    Max Cabin Diff 5 psi
 (This is the only window having a middle pane)
Inner Pane                        Window 1, 2, 5                                Max Cabin Diff 5 psi
Inner Pane                        Window 3, 4                                    Normal Operations
Both Panes                       Windows 1, 2, 4, 5                          Max Cabin Diff 2 psi
Both Panes                        Window 3                                       Max Cabin Diff 0 psi

Max Airspeed with Window Heat Inoperative is 250 kts Below 10,000'