About vector masks
If you have used other vector-illustration applications such as Adobe FreeHand, you may be familiar with vector masks, which are sometimes called clipping paths or paste insides. The vector mask object crops or clips the underlying objects to the shape of its path, creating a cookie-cutter effect.
A vector mask applied using its path outline
When you create a vector mask, a mask thumbnail with a pen icon appears in the Layers panel to indicate you've created a vector mask.
A vector mask thumbnail in the Layers panel
When a vector mask is selected, the Property inspector displays information about how the mask is applied. The bottom half of the Property inspector displays additional properties that allow you to edit the mask object's stroke and fill.
Vector mask properties in the Property inspector
By default, vector masks are applied using their path outline, but you can also apply them in other ways. For more information, see Changing the way masks are applied.