[Fontconfig] target pattern or font?

[BubuXP]

I'm trying to understand the difference between <match target="pattern">
and <match target="font">:
== If 'target' is set to "font" instead of the default "pattern", then this
element applies to the font name resulting from a match rather than a font
pattern to be matched. ==
It should be a clear statement but probably my english (or better my head)
is not good enough to understand. Some examples could help me.

Reading further in the fontconfig official documentation I found this in an
example user-configuration file:

<!-- use rgb sub-pixel ordering to improve glyph appearance
on LCD screens.  Changes affecting rendering, but not
matching should always use target="font". -->
<match target="font">
<edit name="rgba" mode="assign"><const>rgb</const></edit>
</match>

Is that statement valid only for normal users configurations? Because the
system-wide configuration for the same setting (you can found it in
conf.avail/) uses target="pattern".
When one should use "pattern" and when "font"?

Thanks to whom that will try to explain me. :)
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