The 48" x 48" is a mid-size Egress window enabling egress out of the basement living spaces per the International Residential Code (IRC) for use in emergency situations.
The Wellcraft Slider Egress Window is attractive and provides for quick/easy fire escape.
Rule of Thumb: Egress Windows must be installed in below-grade rooms where people will sleep.
By IRC Code: The minimum allowable height for an egress window is 24" and the window must have a minimum of 5.7 sq ft total opening to allow for escape. The 48" x 48" sliding window meets this and allows easy egress out of the residence.