A 90 degree operable skylight lifts the glazed unit or "lid" 90 degrees from the plane of the curb. This type of skylight has different applications and specific functions, one being ventilation.
A typical ridge vent or standard operable skylight only opens a few degrees, allowing small amounts of ventilation. The 90 degree unit fully opens, allowing optimum amounts of ventilation into a space.
Daylight is a desired effect of all skylights, and the 90 degree series allows ample levels of daylight.