The last page you were viewing contains a reference to an internal resource. For users inside NCBI, the reference is a hyperlink. For users outside NCBI, the reference is just bracketed text, with an asterisk that links to this page.
Because the great majority of the NCBI C++ Toolkit book is applicable whether you are inside or outside of NCBI, it would be very inefficient to maintain separate NCBI-internal and public versions. However, if we published web pages with links to internal content, those pages would contain broken links for users outside NCBI. Therefore, for outside users, we simply replace all internal links with bracketed text and a link to this page.