The special $UBook tag is used once, at the very start of the manuscript to identify the content to book reader software.
How it works
Add the tag at the very beginning of the manuscript, before anything else.
UBook language designed by gdoc.
It is requested that you copy and paste all of the above at the top of your manuscripts. The comment about the UBook language is not actually needed, but it is hoped you will leave it intact as a courtesy.