The FontSet element defines a group of fonts to be used in cases where it is desirable to support more characters than exist in a single font (also referred to as "fallback fonts" support).

For example, on a layer with both English and Chinese names, one could specify fonts with a high level of legibility for English letters as well as a fallback font for Chinese characters that don't exist in the English font.

Here's a small example:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE Map>
<Map srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
 <FontSet name="my-fonts">
  <Font face_name="DejaVu Sans Book" />
  <Font face_name="TSC FMing S TT Regular" />
 </FontSet>

 <Style name="font-test">
  <Rule>
   <TextSymbolizer name="NAME" fontset_name="my-fonts" size="15" fill="black" />
  </Rule>
 </Style>

 <Layer name="font-test" status="on" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
  <StyleName>font-test</StyleName>
   <Datasource>
    <Parameter name="type">shape</Parameter>
    <Parameter name="file">Font_Test</Parameter>
   </Datasource>
  </Layer>
</Map>