Creating A Mask From An Existing Object; Masking Objects Using The Paste As Mask Command - MACROMEDIA FIREWORKS 8-USING FIREWORKS Use Manual

When you create a bitmap mask, the Property inspector displays information about how the
mask is applied. If you select a bitmap tool when a bitmap mask is selected, the Property
inspector displays the mask's properties and options for the selected tool, simplifying the
mask-editing process.
Bitmap mask properties in the Property inspector when a bitmap tool is selected
By default, most bitmap masks are applied using their grayscale appearance, but you can also
apply them using their alpha channel. For more information, see "Changing the way masks
are applied" on page 227.

Creating a mask from an existing object

You can create a mask from an existing object. When used as a mask, a vector object's path
outline can be used to clip or crop other objects. When a bitmap object is used as a mask,
either the brightness of its pixels or its transparency affects the visibility of other objects.

Masking objects using the Paste as Mask command

Using the Paste as Mask command, you can create masks by masking an object or group of
objects with another object. Paste as Mask creates either a vector mask or a bitmap mask.
When you use a vector object as the mask, Paste as Mask creates a vector mask that crops or
clips masked objects using the path outline of the vector object. When you use a bitmap
image as the mask, Paste as Mask creates a bitmap mask that affects the visibility of masked
objects using the grayscale color values of the bitmap object.
Masking images 213