=> label (container | heading)*
@end example
-The root of a structure member is a @code{heading}, which represents a
-section of output beginning with a @code{label} and
-ordinarily followed by content containers or further nested
-(sub)-sections of output. Unlike heading elements in HTML and other
-common document formats, which precede the content that they head,
-@code{heading} contains the elements that appear below the heading.
-
-The document root heading, only, may contain a @code{pageSetup}
-element.
+A @code{heading} represents a tree of content that appears in an
+output viewer window. It contains a @code{label} text string that is
+shown in the outline view ordinarily followed by content containers or
+further nested (sub)-sections of output. Unlike heading elements in
+HTML and other common document formats, which precede the content that
+they head, @code{heading} contains the elements that appear below the
+heading.
+
+The root of a structure member is a special @code{heading}. The
+direct children of the root @code{heading} elements in all structure
+members in an SPV file are siblings. That is, the root @code{heading}
+in all of the structure members conceptually represent the same node.
+The root heading's @code{label} is ignored (see @pxref{SPV Structure
+label Element}). The root heading in the first structure member in
+the Zip file (typically named @file{outputViewer0000000000.xml}) may
+contain a @code{pageSetup} element.
The following attributes have been observed on both document root and
nested @code{heading} elements.
@end defvr
@defvr {Attribute} @code{visibility}
-To what degree the output represented by the element is visible.
+If this attribute is absent, the heading's content is expanded in the
+outline view. If it is set to @code{collapsed}, it is collapsed.
+(This attribute is never present in a root @code{heading} because the
+root node is always expanded when a file is loaded, even though the UI
+can be used to collapse it interactively.)
@end defvr
@defvr {Attribute} @code{locale}
The text in @code{label} describes what it labels, often by naming the
statistical procedure that was executed, e.g.@: ``Frequencies'' or
-``T-Test''. The root @code{heading} in a structure member is normally
-``Output''. Labels are often very generic, especially within a
+``T-Test''. Labels are often very generic, especially within a
@code{container}, e.g.@: ``Title'' or ``Warnings'' or ``Notes''.
Label text is localized according to the output language, e.g.@: in
Italian a frequency table procedure is labeled ``Frequenze''.
contains a few examples of empty labels, ones that contain no text,
probably as a result of user editing.
+The root @code{heading} in an SPV file has a @code{label}, like every
+@code{heading}. It normally contains ``Output'' but its content is
+disregarded anyway. The user cannot edit it.
+
@node SPV Structure container Element
@subsection The @code{container} Element
@defvr {Attribute} @code{visibility}
Whether the container's content is displayed. ``Notes'' tables are
-often hidden; other data is usually
+often hidden; other data is usually visible.
@end defvr
@defvr {Attribute} @code{text-align}