请教一个字体配置文件里面的问题
很多配置文件里面都有：<!--
Serif faces
-->
<alias>
<family>Bitstream Vera Serif</family>
<family>Times</family>
<family>Times New Roman</family>
<family>Nimbus Roman No9 L</family>
<family>Luxi Serif</family>
<family>SimSun</family>
<family>PMingLiu</family>
<family>Kochi Mincho</family>
<family>AR PL SungtiL GB</family>
<family>AR PL Mingti2L Big5</family>
<family>Baekmuk Batang</family>
<default><family>serif</family></default>
</alias>
<!--
Sans-serif faces
-->
<alias>
<family>Bitstream Vera Sans</family>
<family>Helvetica</family>
<family>Arial</family>
<family>Verdana</family>
<family>Nimbus Sans L</family>
<family>Luxi Sans</family>
<family>SimSun</family>
<family>PMingLiu</family>
<family>Kochi Gothic</family>
<family>AR PL KaitiM GB</family>
<family>AR PL KaitiM Big5</family>
<family>Baekmuk Dotum</family>
<default><family>sans-serif</family></default>
</alias>
<!--
Monospace faces
-->
<alias>
<family>Bitstream Vera Sans Mono</family>
<family>Courier</family>
<family>Courier New</family>
<family>Andale Mono</family>
<family>Luxi Mono</family>
<family>Nimbus Mono L</family>
<family>NSimSun</family>
<family>MingLiu</family>
<default><family>monospace</family></default>
</alias>
<!--
Provide required aliases for standard names
-->
<alias>
<family>serif</family>
<prefer>
<family>Bitstream Vera Serif</family>
<family>Times New Roman</family>
<family>Nimbus Roman No9 L</family>
<family>Luxi Serif</family>
<family>Times</family>
<family>SimSun</family>
<family>Kochi Mincho</family>
<family>PMingLiu</family>
<family>AR PL SungtiL GB</family>
<family>AR PL Mingti2L Big5</family>
<family>Baekmuk Batang</family>
</prefer>
</alias>
<alias>
<family>sans-serif</family>
<prefer>
<family>Bitstream Vera Sans</family>
<family>Verdana</family>
<family>Nimbus Sans L</family>
<family>Luxi Sans</family>
<family>Arial</family>
<family>Helvetica</family>
<family>SimSun</family>
<family>PMingLiu</family>
<family>AR PL KaitiM GB</family>
<family>AR PL KaitiM Big5</family>
<family>Kochi Gothic</family>
<family>Baekmuk Dotum</family>
</prefer>
</alias>
<alias>
<family>monospace</family>
<prefer>
<family>Bitstream Vera Sans Mono</family>
<family>Andale Mono</family>
<family>Courier New</family>
<family>Luxi Mono</family>
<family>Nimbus Mono L</family>
<family>NSimSun</family>
<family>MingLiu</family>
<family>AR PL KaitiM GB</family>
<family>Kochi Gothic</family>
<family>Baekmuk Dotum</family>
</prefer>
</alias>
请问这里面的定义是什么意思？起什么作用呢？
根据注释<!-- Serif faces -->和<!-- Provide required aliases for standard names-->下面的定义有什么区别呢？
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