<depth> contains a measurement measured across the spine of a book or codex, or (for other text-bearing objects) perpendicular to the measurement given by the ‘width’ element. [10.3.4 Dimensions]
If used to specify the width of a non text-bearing portion of some object, for example a monument, this element conventionally refers to the axis facing the observer, and perpendicular to that indicated by the ‘width’ axis.