Layers
Layers let you organize your painting. They work on a per-page basis: every page of the notebook has its own layers, and they are not shared between pages.
Layer limit
You can create a maximum of 24 layers per page.
When you get close to that limit, you'll notice the painting gets a little slower.
Layer features
Opacity
You can adjust the transparency of the selected layer.
Blend modes
Blend modes change how the layer you have active combines with the ones below it.
The control is called Blend and offers 16 modes, from the most basic (Normal, Multiply, Screen, Overlay, Darken, Lighten) through the ones that change the hue, saturation, color or luminosity of what is below. It applies to the active layer, or to every layer you have selected at once.
Duplicate layer
Creates a copy of the selected layer, right above it, with all its content. Very useful for retouching an area without worrying about touching the original.
Move layer
You can move the layer along the list to change its position, either by dragging it or with the move up and move down buttons.
Show / Hide
The layer stops being visible, but it is not deleted: it still exists.
Delete
Deletes the layer and its content.
Lock
Prevents the contents of the locked layer from being modified or moved.
Lock transparent pixels
You can only paint on the areas of the selected layer where there is already something painted, not on the transparent areas.
Select multiple layers
Every layer has a checkbox on its left. By checking several, all layer actions apply to the whole selection at once.
Paste image from clipboard
Pastes the image in the system clipboard onto a new layer.
Apply Image (Flatten)
Merges all the layers into a new layer as a single image.
Merge Down
Merges the selected layer with the layer below it.
Flatten All
Combines all the layers into a single layer. Unlike the "Apply Image" option, this one is destructive: it deletes every layer except the new one.
Delete Selected
Deletes all the layers you have checked in the selection box.