Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 181

• Add Layer Mask : You can add a mask on top of a layer to select just partial
areas of the layer in order to edit the layer's elements without changing
the layer itself. A layer mask is directly assigned to the selected layer, but
it can be edited separately as a black-and-white channel or grayscale
image ( Edit Layer Mask ).
• Apply Layer Mask : Once a layer mask has been edited and checked, its
effect can be applied to the relevant layer. The layer mask itself is deleted
and the editing is directly applied to the pixels of the layer. Don't use this
menu item if you want to keep the layer mask for further editing.
• Delete Layer Mask : Deletes a layer mask, discarding the changes you
made to the relevant layer.
• Show Layer Mask : Makes a layer mask visible.
• Edit Layer Mask : Allows editing of the layer mask. Your changes will be
displayed in the Show Layer Mask dialog. If you don't use this dialog, the
changes to the mask will be visible in the image object or in the layer
content itself.
• Disable Layer Mask : Disables a layer mask or its effect on the layer
without deleting the layer mask itself.
• Mask to Selection : Transforms the active layer's mask into a selection.
For example, if you created a mask with specific border attributes (that
means sharp edges or feathering), you can copy the mask's attributes to
a new selection.
• Add Alpha Channel : This item is available only for background layers
without transparency, i.e., without an alpha channel. Adding an alpha
channel transforms a background layer into a normal layer, enabling you
to use transparency on the layer as well as to move the layer in the Layers
dialog.
• Remove Alpha Channel : Deletes the alpha channel of a layer so that the
layer isn't transparent but the opacity can still be adjusted. The name of
the layer appears in bold in the Layers dialog.
• Alpha to Selection : Use this option to easily create a selection based
on an existing object and including the feathering and/or transparency
attributes assigned to that object.
• Merge Visible Layers : Reduces all visible layers on a normal layer with
alpha transparency.
• Flatten Image : This option reduces all layers of an image to one single
background layer (no alpha channel [i.e., transparency]).

• NOTE
Even if you duplicate a background
layer, it does not automatically add
an alpha channel.