Etching is a way of making the icon appear to be part of the surface it is etched on. Not all the parts of an icon have to be etched into the surface. You can apply selective etching, making part of the object anchor itself in the panel and some of it lie on the panel or protrude from it slightly.
The Help icon, for example, takes away the etch, made with the topShadow color, under and next to the right-hand book, and replaces it with a select color shadow. This makes it appear that one book is protruding slightly from the shelf. The printer icon has a protruding paper tray. In the Style Manager icon, the palette, letters, and mouse are above the etched-in window frame. The File Manager icon has gone the furthest, as only the edge of the opening is etched, while the drawer front and the cocked folder protrude and even have a shadow.
The principle is to have something in the artwork anchored, yet let the 3-D nature of the objects come out as well. The variable content in an icon, like the printer page or the mail envelopes in the Mailer icon, should not be anchored.