A piece of Scenery is a map object providing backstory to the map or just giving a good look to the map. These scenery items are put in 5 different categories: Lava (L), Water (W), Sewage (S), Jjaro (J) and Pfhor (P) just like textures.
Scenery can be used to direct the player to it's goal like a path of blood drops. Or to hint at the backstory of the location, like Jjaro statues, Pfhor bodies, skulls, etc. Or for making the map look better, i.e. lamps, antennas, trash cans. For instance; in Rise Robot Rise (Level) there's a pit filled with Pfhor pieces and Hunter shields. This is an indication that this pit is bad news, because once you step in it it fills with goo (you can get out though). It also indicates that there were other Pfhor that died in this pit as well.