How to deal with Tag

[LI Daobing <lidaobing@gmail.com>]

Hello,
    When I use `apt-cache show xpdf', I can find a `Tag' entry. But I
don't know how to add tag entry when I build a debian package. I did
not find any `Tag' in the debian directory of the source package.
    Another question, How to find all-packages that has a tag `format::pdf'?

Thanks
-- 
LI Daobing