The nonPositiveInteger datatype is derived from the integer type : nonPositiveInteger « XML Schema « XML Tutorial

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3. nonPositiveInteger

The value space is the infinite set of integers less than and including zero. 
The lexical space is a finite-length sequence of decimal digits preceded by a -.

<!-- schema -->
<xsd:simpleType name="BC">
  <xsd:restriction base="xsd:nonPositiveInteger">
   <xsd:minInclusive value="unbounded"/>
   <xsd:maxInclusive value="-2001"/>
  </xsd:restriction>
</xsd:simpleType>
<!-- instance document -->
<BC>-1111</BC>