-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.