db._phrase

From the on-line description of db._phrase 
(http://www.docbook.org/tdg5/en/html/_phrase.html):
 > This variant of phrase is used in places where
 > the content model is restricted to only the
 > “ubiquitous” inlines.

Can someone explain to me what this means, and why it is necessary to 
have both db.phrase and db._phrase in DB5?  Unless I'm missing 
something, it was not necessary in earlier versions of DB (it doesn't 
appear in the "DocBook: The Definitive Guide" v1.0.3, although that is 
of course an older version).  (There is also a db._emphasis, in addition 
to the db.emphasis, although it's not clear that db._emphasis exists for 
the same reason as db._phrase--the '"ubiquitous" inlines isn't mentioned 
in the same way.)
-- 
    Mike Maxwell
    What good is a universe without somebody around to look at it?
    --Robert Dicke, Princeton physicist