+The design of the detail XML format is not what one would end up with
+for describing pivot tables. This is because it is a specialization
+of a much more general format (``visualization XML'' or ``VizML'')
+that can describe a wide range of visualizations. Most of this
+generality is overkill for tables, and so we end up with a funny
+subset of a general-purpose format.
+
+The important elements of the detail XML format are:
+
+@itemize @bullet
+@item
+Variables. Variables in detail XML roughly correspond to the
+dimensions in a light detail member. There is one variable for each
+dimension, plus one variable for each level of labeling along an axis.
+
+The bulk of variables are defined with @code{sourceVariable} elements.
+The data for these variables comes from the associated
+@code{tableData.bin} member. Some variables are defined, with
+@code{derivedVariable} elements, as a constant or in terms of a
+mapping function from a source variable.
+
+@item
+Assignment of variables to axes. A variable can appear as columns, or
+rows, or layers. The @code{faceting} element and its sub-elements
+describe this assignment.
+@end itemize
+
+All elements have an optional @code{id} attribute. In practice many
+elements are assigned @code{id} attributes that are never referenced.