xpanning allows you to pan, or shift, the image of a sprite the same way you would pan a ceiling, floor, or wall texture along the x axis. Values are normalized on a 0-255 scale, meaning that regardless of the sprite's size, a value of 128 will pan it 50%.
The CON wall.xpanning refers to the struct member of the wall structure.
The CON actor.xpanning field refers to the struct member of the spriteext structure (the sprite structure doesn't have such a member). Since spriteext is only used in the OpenGL renderers, it has no effect on classic mode; moreover, spriteext xpanning is only implemented for Polymost as of December 2011. When used on a sprite with a model assigned to it in Polymost mode, xpanning controls the model's texture UV map position.