What does a typographer mean, when he is talking about tagging?
Usually we mean to emphasis a letter or a word or more words in a text.
So tagging is quite simple: when you want a word to be bold or italic, you are tagging it. So a Tag is a variance of your bodyfont. To be able to use tagging properly, it is quite important that your font has a great variety in font styles.
Some examples of tagging:
Certainly it is important that an author tells the layouter or mediadesigner, which words should be tagged. And it is also important to use tagging carefully. Underlining is a tagging which is no longer that popular, coz in times of good old typewriters it was the only way of tagging.