The $ST tag is used only on the title page to indicate the subtitle of the book.
How it works
$ST is added before the subtitle with a space.
// Title page
$T His Last Bow
$ST A Reminiscence of Sherlock Holmes
$A Arthur Conan Doyle
$P John Murray Publishers, London, England
$PB
$T His Last Bow
$ST A Reminiscence of Sherlock Holmes
$A Arthur Conan Doyle
$P John Murray Publishers, London, England
$PB