Example custom XML configuration files for the Mage2Katas episode 9: The Custom Config File Kata.

unit_conversion.xml

<?xml version="1.0"?>
<conversion_map xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Mage2Kata_DiConfig:etc/unit_conversion.xsd">
    <unit id="mg" type="weight">
        <conversion to="g" factor="0.001000"/>
        <conversion to="kg" factor="0.000001"/>
        <conversion to="lbs" factor="0.00000220462257811"/>
    </unit>
    <unit id="g" type="weight">
        <conversion to="mg" factor="1000"/>
        <conversion to="kg" factor="0.001"/>
        <conversion to="lbs" factor="0.00220462257811"/>
    </unit>
    <unit id="lbs" type="weight">
        <conversion to="mg" factor="453592.379"/>
        <conversion to="g" factor="453.592379"/>
        <conversion to="kg" factor="0.453592379"/>
    </unit>
    <unit id="kg" type="weight">
        <conversion to="mg" factor="1000000"/>
        <conversion to="g" factor="1000"/>
        <conversion to="lbs" factor="2.20462257811"/>
    </unit>
    <unit id="ml" type="fluid">
        <conversion to="l" factor="0.001"/>
    </unit>
    <unit id="l" type="fluid">
        <conversion to="ml" factor="1000"/>
    </unit>
    <unit id="mm" type="distance">
        <conversion to="cm" factor="0.1"/>
        <conversion to="in" factor="0.0393700787402"/>
        <conversion to="m" factor="0.001"/>
    </unit>
    <unit id="cm" type="distance">
        <conversion to="mm" factor="10"/>
        <conversion to="in" factor="0.393700787402"/>
        <conversion to="m" factor="0.01"/>
    </unit>
    <unit id="in" type="distance">
        <conversion to="mm" factor="25.4"/>
        <conversion to="cm" factor="2.54"/>
        <conversion to="m" factor="0.0254"/>
    </unit>
    <unit id="m" type="distance">
        <conversion to="mm" factor="1000"/>
        <conversion to="cm" factor="100"/>
        <conversion to="in" factor="39.3700787402"/>
    </unit>
</conversion_map>

unit_conversion.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="conversion_map" type="conversion_mapType">
    <xs:unique name="uniqueUnit">
      <xs:selector xpath="unit" />
      <xs:field xpath="@id" />
    </xs:unique>
  </xs:element>
  <xs:complexType name="conversion_mapType">
    <xs:sequence>
      <xs:element type="unitType" name="unit" maxOccurs="unbounded" minOccurs="0">
        <xs:unique name="uniqueTargetUnit">
          <xs:selector xpath="conversion" />
          <xs:field xpath="@to" />
        </xs:unique>
      </xs:element>
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="unitType">
    <xs:sequence>
      <xs:element type="conversionType" name="conversion" maxOccurs="unbounded" minOccurs="1"/>
    </xs:sequence>
    <xs:attribute type="unitId" name="id" use="required"/>
    <xs:attribute type="measurementType" name="type" use="required"/>
  </xs:complexType>
  <xs:complexType name="conversionType">
    <xs:simpleContent>
      <xs:extension base="xs:string">
        <xs:attribute type="targetUnitType" name="to" use="required"/>
        <xs:attribute type="factorType" name="factor" use="required"/>
      </xs:extension>
    </xs:simpleContent>
  </xs:complexType>
  <xs:simpleType name="unitId">
    <xs:restriction base="xs:string">
      <xs:pattern value="[a-z]+" />
    </xs:restriction>
  </xs:simpleType>
  <xs:simpleType name="targetUnitType">
    <xs:restriction base="xs:string">
      <xs:pattern value="[a-z]+" />
    </xs:restriction>
  </xs:simpleType>
  <xs:simpleType name="measurementType">
    <xs:restriction base="xs:string">
      <xs:enumeration value="weight"/>
      <xs:enumeration value="fluid"/>
      <xs:enumeration value="distance"/>
    </xs:restriction>
  </xs:simpleType>
  <xs:simpleType name="factorType">
    <xs:annotation>
      <xs:documentation>
        Factor to convert base unit to target unit
      </xs:documentation>
    </xs:annotation>
    <xs:restriction base="xs:decimal">
      <xs:minExclusive value="0"/>
    </xs:restriction>
  </xs:simpleType>
</xs:schema>