In Harmony, everything that constitutes the different elements of a scene, including drawing layers, sound effects, visual effects and pegs is represented by a layer in the Timeline view.
At the core of the rendered picture is the drawing layer, which is used to display drawings. Those layers are rendered from bottom to top, with the bottommost layer appearing in the background, and the topmost layer appearing in the foreground.
Some layers, such as Peg, Bone or Quadmap layers, are made to offset drawing layers. They must be created as the parent of a drawing layer, or of a hierarchy of drawing layers, in order to transform the layers in the rendered image.
Other layers, such as Blur and Transparency layers, are made to transform the picture output by drawing layers. They must be created as children of drawing layers, or groups of drawing layers, in order to transform the drawings.