In construction shorthand, usually means "Surfaced 4 Sides" (dimensional lumber). However, in the context of this defect, S4U often refers to a specific architectural detail or a user-defined location tag—specifically the top termination point of the stair stringer against a vertical drywall or partition.
When utilizing the s4u Stair Plugin to build parametric steps from a series of flat faces or continuous curves, SketchUp relies heavily on clean profile edges. Open gaps or missing faces at the top transition happen because of three main system limitations: s4u stair crack top