The safety of the capacitor
The principle is that if a voltage is impressed across the film and the film's dielectric strength limit is exceeded, the film will be punctured at its weakest point but the material in the punctured region won't char and short the plates, it'll evaporate and "fail open" in that region, leaving the rest of the film undisturbed.

This feature is called "self-healing", and is what makes a capacitor comprising the metalized film "safe highly."