A sidedef contains the wall texture data for each linedef (though sidedefs do not reference linedefs directly, indeed it is the other way around). Each sidedef contains texture data, offsets for the textures and the number of the sector it references (this is how sectors get their 'shape'). Upper texture - This contains the name of a texture that will be displayed on the border between a sector and its neighboring ceiling of a different height. If the linedef that contains this sidedef is one sided this field is meaningless.