Skylight Constraint Relative to Roof

either Horizontally (the Skylight will retain its position as seen on the Floor Plan)
or Vertically (the Skylight will retain its vertical elevation regardless of its Floor Plan position.)
Change this Constraint in Roof setting in Skylight Settings or the Info Box.
Use the Horizontal constraint if you want the Skylight to remain in the same place when viewed from above (that is, on the Floor Plan) even after the host Roof plane geometry changes. This option is useful if your Skylight is the kind that lets in light from the top of the building.
Use the Vertical constraint if you want the Skylight to remain at a constant elevation even after the host Roof plane geometry changes. This option is useful if you need a Skylight at a particular vertical position (e.g. as a lookout window in the attic).