The Layer system is very important for composing a photo. A layer means an individual image where the user can apply processing over it while keeping other layers unchanged.
The first layer is called the 'base layer' and is a fixed, opaque layer which decides the size of the whole canvas. The layers over the base layer are 'float layers' which have such properties:
Offset: The position of this layer. Using a 'move' tool to change it,
Visible: Show or hide a layer.
Opacity: from 0-100 to define the layer from transparent to solid.
Blending mode: include normal, darken, lighten, multiply, screen, overlay, hard light, soft light and difference.