A puzzlescript file is divided into 8 sections:
Objects can be placed on different layers:
Background Target Player, Wall, Crate
The main purpose of layers is in resolving movements: two objects on the same layer cannot coexist in a single cell. The order of layers also determines in what order things are drawn.
The Background layer is a special layer. Every game must have one. Every tile must have a background tile. However, by defining
Background = Background1 or Background2
in the legend, you can have several tile types on the background layer. Every tile must have a background tile. If you don't specify one explicitly for a tile, it is inferred based on what other background tiles are used in the level it's in.