Users may at times require windowing of the soldermask layer to expose a copper
track.
<p>
The layer preferences dialog provides a visual stack up of the layers in a design.
The user can right click on a layer to add an additional layer, and in the layer
creation dialog window the user is given the opportunity to nominate the new layer
as one in which "drawing with subtraction" is used.
<p>
Layer groups with multiple layers in them are called <i>composite</i>,
regardless of the layers being positive or negative. If a layer group's first
layer is a negative layer, an implicit all-board positive fill is 
automatically applied before the first layer.
<p>
With this type of layer, any drawn feature is subtracted from other features. This
allows complex windowing of existing features on any layer type.
<p>
Note: subtraction is not available on copper layers.