Fontconfig - Dec 2006 - i have some question, who can tell me

Here have a definition in font.conf:
<alias>
  <family></family>
  <prefer></prefer>
  <accept></accept>
  <default></default>
</alias>
and
  <prefer></prefer>
  <accept></accept>
  <default></default>
is choice,I still wander what does it mean though I read manual?who can tell
me the setting and tag do for what?
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