In context of having <fo:block> with multiple font families listed, like:

<block font-family='serif,Symbol'>
  (∀1) (+2) (↑3) (+4) (∀5)
</block>

or

<block font-family='serif,Symbol'>
  (∀1) (+2) (<character character='↑'/>3) (+4) (∀5)
</block>


from http://mail-archives.apache.org/mod_mbox/xmlgraphics-fop-users/200808.mbox/%3c1220224006.15155.13.camel@moonpix%3e

"""
'Symbol' is a taller font than 'serif'.

FOP decides how tall each line should be and then places characters
within that line.  In the event that a character is "too tall" to fit
then it ends up being aligned to the top (and hanging out the bottom).
This is what happens when you have UTF8 character inline.

When you use <character> you somehow get FOP to notice that you're
placing taller characters in the line and it takes this into account
when deciding the proper height of the line.

When you don't use <character> then FOP makes its decision about the
height of the line based solely on the first listed font family
(ignoring all of the others, irrespective of if they are used for font
substitution in that line).

"""

Andreas Delmelle provides addition insight in his email here: http://mail-archives.apache.org/mod_mbox/xmlgraphics-fop-users/200809.mbox/%3c12DB9901-D56D-4C4C-A77B-CF3A35C4EE5E@telenet.be%3e

"""
Having taken a quick, closer look at the related code, it goes in  
this direction indeed.

Technically, the story is that, without fo:character or fo:inline a  
combined text-area is generated for each separate 'word' (in the  
sense of: uninterrupted sequence of non-white-space characters,  
regardless of whether they can be rendered in the same font).

Those areas are currently all based on a single alignment-context  
(which seems to correspond to the first font-family in the list; this  
explains why we get a different result when putting the Symbol font  
first). AFAICT, it does not seem like a real tough problem to  
solve... I do seem to remember Max pointing out this issue at some  
time while implementing font-selection (?)
If we place the characters in an fo:inline or an fo:character, the  
only big difference is that a new alignment-context is created  
automatically, which later on triggers correct baseline alignment of  
the two pieces.
"""