+@defvr {Required} source
+Always set to @code{tableData}, the @code{source-name} in the
+corresponding @file{tableData.bin} member (@pxref{SPV Legacy Member
+Metadata}).
+@end defvr
+
+@defvr {Required} sourceName
+The name of a variable within the source, the @code{variable-name} in
+the corresponding @file{tableData.bin} member (@pxref{SPV Legacy
+Member Data}).
+@end defvr
+
+@defvr {Optional} dependsOn
+The @code{variable-name} of a variable linked to this one, so that a
+viewer can work with them together. For a group variable, this is the
+name of the corresponding categorical variable.
+@end defvr
+
+@defvr {Optional} label
+The variable label, if any
+@end defvr
+
+@defvr {Optional} labelVariable
+The @code{variable-name} of a variable whose string values correspond
+one-to-one with the values of this variable and are suitable for use
+as value labels.
+@end defvr
+
+@node SPV Detail derivedVariable Element
+@subsection The @code{derivedVariable} Element
+
+Parent: @code{visualization} @*
+Contents: @code{extension}* (@code{format} @math{|} @code{stringFormat} @code{valueMapEntry}*)
+
+Like @code{sourceVariable}, this element defines a variable whose
+values can be used elsewhere in the visualization. Instead of being
+read from a data source, the variable's data are defined by a
+mathematical expression.
+
+This element has the following attributes.
+
+@defvr {Required} categorical
+Always set to @code{true}.
+@end defvr
+
+@defvr {Required} value
+An expression that defines the variable's value. In theory this could
+be an arbitrary expression in terms of constants, functions, and other
+variables, e.g.@: @math{(@var{var1} + @var{var2}) / 2}. In practice,
+the corpus contains only the following forms of expressions:
+
+@table @code
+@item constant(@var{number})
+@itemx constant(@var{variable})
+A constant. The meaning when a variable is named is unknown.
+Sometimes the ``variable name'' has spaces in it.
+
+@item map(@var{variable})
+Transforms the values in the named @var{variable} using the
+@code{valueMapEntry}s contained within the element.
+@end table
+@end defvr
+
+@defvr {Optional} dependsOn
+The @code{variable-name} of a variable linked to this one, so that a
+viewer can work with them together. For a group variable, this is the
+name of the corresponding categorical variable.
+@end defvr
+
+@menu
+* SPV Detail valueMapEntry Element::
+@end menu
+
+@node SPV Detail valueMapEntry Element
+@subsubsection The @code{valueMapEntry} Element
+
+Parent: @code{derivedVariable} @*
+Contents: empty
+
+A @code{valueMapEntry} element defines a mapping from one or more
+values of a source expression to a target value. (In the corpus, the
+source expression is always just the name of a variable.) Each target
+value requires a separate @code{valueMapEntry}. If multiple source
+values map to the same target value, they can be combined or separate.
+
+@code{valueMapEntry} has the following attributes.
+
+@defvr {Required} from
+A source value, or multiple source values separated by semicolons,
+e.g.@: @code{0} or @code{13;14;15;16}.
+@end defvr
+
+@defvr {Required} to
+The target value.
+@end defvr
+
+@node SPV Detail extension Element
+@subsection The @code{extension} Element
+
+This is a general-purpose ``extension'' element. Readers that don't
+understand a given extension should be able to safely ignore it. The
+attributes on this element, and their meanings, vary based on the
+context. Each known usage is described separately below. The current
+extensions use attributes exclusively, without any nested elements.
+
+@subsubheading @code{visualization} Parent Element
+
+With @code{visualization} as its parent element, @code{extension} has
+the following attributes.
+
+@defvr {Optional} numRows
+An integer that presumably defines the number of rows in the displayed
+pivot table.
+@end defvr
+
+@defvr {Optional} showGridline
+Always set to @code{false} in the corpus.
+@end defvr
+
+@defvr {Optional} minWidthSet
+@defvrx {Optional} maxWidthSet
+Always set to @code{true} in the corpus.
+@end defvr
+
+@subsubheading @code{container} Parent Element
+
+With @code{container} as its parent element, @code{extension} has the
+following attributes.
+
+@defvr {Required} combinedFootnotes
+Always set to @code{true} in the corpus.
+@end defvr
+
+@subsubheading @code{sourceVariable} and @code{derivedVariable} Parent Element
+
+With @code{sourceVariable} or @code{derivedVariable} as its parent
+element, @code{extension} has the following attributes. A given
+parent element often contains several @code{extension} elements that
+specify the meaning of the source data's variables or sources, e.g.@:
+
+@example
+<extension from="0" helpId="corrected_model"/>
+<extension from="3" helpId="error"/>
+<extension from="4" helpId="total_9"/>
+<extension from="5" helpId="corrected_total"/>