toc

      CONTENTS or TOC
         The TOC relationship identifies a Table of Contents.

         When REL=TOC, the target document is the Table of Contents for
         the current document, or for the collection of documents of
         which the current document is a member.

         When REV=TOC, the current document is a Table of Contents and
         the target document is a related document.

         When REL=TOC and REV=TOC it indicates that the current
         document is a Table of Contents and the target document is
         also a Table of Contents.  Additional REL/REV values may be
         used to specify the relationship between the two, such as
         PARENT/CHILD.

         If the hypertext link is specified with REL in a LINK element,
         an HTML user agent may present an icon in a tool bar. Or, if
         capable, an HTML user agent may present the Table of Contents
         in a concurrent window or pane, highlighting the current
         document.

REL=ToC

The link references a document serving as a table of contents.