The unsignedByte data type is derived from the unsignedShort type. : unsignedByte « XML Schema « XML Tutorial

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3. unsignedByte

The value space is the set of integers greater than or equal to 0 and less than or equal to 255. 
The lexical space is a finite-length sequence of decimal digits.

<!-- schema -->
<xsd:complexType name="RGB">
  <xsd:sequence>
   <xsd:element name="red" type="xsd:unsignedByte"/>
   <xsd:element name="green" type="xsd:unsignedByte"/>
   <xsd:element name="blue" type="xsd:unsignedByte"/>
  </xsd:sequence>
</xsd:complexType>

<!-- instance document -->
<red>33</red>
<green>255</green>
<blue>99</blue>