F_SKY1 is the name of a special flat used to render skies above—or sometimes below—sectors. It is not a true flat; while there is a lump named
F_SKY1 in the IWAD, the engine considers it a signal to the renderer that the appropriate sky textures should be rendered where ceiling/floor flats otherwise would be.
Note that, while a map's
SkyNum might for example be 29 (Spring Hill Zone's sky), as defined in the level header, the texture on the sector's ceiling or floor should still be specified as
F_SKY1, and not the corresponding
SKYnn texture: it is the game's responsibility to determine exactly which sky should be rendered. This allows skies to be changed dynamically in the course of a level by means of linedef type 423.
F_SKY1 on their ceilings or floors also cause slightly different rendering behavior for upper or lower textures on adjacent sidedefs, respectively. Since the sky appears to extend infinitely upwards or downwards, upper and lower textures would look out of place. Because of this, they are not rendered even if the sector heights would cause them to be in normal circumstances.
Versions of SRB2 earlier than v2.0 lacked this lump entirely; because of this, many map editors would report that
F_SKY1 was an unknown flat, although it would still behave correctly. v2.0 and later versions include a flat with this name, much like the original Doom games did, effectively ending this error.