The safety box is not really a page box in the strict sense of the word: it is not a page box type which is stored in the PDF document. The safety box is a visual area which is a bit smaller than the trim box. It contains all text, or other objects, which must always be visible on the printed and finished document. The area between the safety box and the trim box is called the safe type zone: this is the zone where you must not place any text or objects, unless these objects are intended as bleed.
The safety box is intended to compensate for incorrect finishing: