What is a daylight envelope?

A Daylight Envelope represents all possible planes which can theoretically daylight from a given slope plane.  

In practice, for planar failure the daylight envelope is overly conservative, and lateral limits can be placed on the allowable dip direction.  Typically, it is assumed that failure planes should have a dip direction within about 20 degrees of the slope direction in order for planar sliding to occur.  You can simply add two lines to the stereonet plot in order to place limits within the daylight envelope (use the Tools>Add Line Option).