The byte data type is derived from the short type. : byte « XML Schema « XML Tutorial

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The value space is integer values from 128 up to and including 127. 
The lexical space is a finite-length sequence of decimal digits with an optional leading - or +.

<!-- schema -->
<xsd:complexType name="myBytes">
  <xsd:choice>
   <xsd:element name="byte1" type="xsd:byte"/>
   <xsd:element name="byte2" type="xsd:byte"/>
   <xsd:element name="byte3" type="xsd:byte"/>
  </xsd:choice>
</xsd:complexType>
<!-- instance document -->
<byte2>55</byte2>