あなたは間違った場所であなたの選択をしています
<?xml version="1.0" encoding="utf-16"?>
<xs:schema xmlns="http://Scratch.ABC" targetNamespace="http://Scratch.ABC" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="Root">
<xs:complexType>
<xs:sequence>
<xs:element minOccurs="1" maxOccurs="1" name="AorBorC" type="group1" />
<xs:element minOccurs="1" maxOccurs="unbounded" name="ABCOrMore" type="group1" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:complexType name="group1">
<xs:choice>
<xs:element minOccurs="0" maxOccurs="1" name="a" type="xs:string" />
<xs:element minOccurs="0" maxOccurs="1" name="b" type="xs:string" />
<xs:element minOccurs="0" maxOccurs="1" name="c" type="xs:string" />
</xs:choice>
</xs:complexType>
</xs:schema>
XML
<ns0:Root xmlns:ns0="http://Scratch.ABC">
<AorBorC>
<a>a_1</a>
</AorBorC>
<ABCOrMore>
<a>a_2</a>
</ABCOrMore>
<ABCOrMore>
<b>b_1</b>
</ABCOrMore>
</ns0:Root>
または、必要に応じて
<ns0:Root xmlns:ns0="http://Scratch.ABC">
<AorBorC>
<a>a_2</a>
</AorBorC>
<ABCOrMore>
<a>a_2</a>
<c>c_1</c>
<b>b_1</b>
</ABCOrMore>
</ns0:Root>
それで
<?xml version="1.0" encoding="utf-16"?>
<xs:schema xmlns="http://Scratch.ABC" xmlns:b="http://schemas.microsoft.com/BizTalk/2003" targetNamespace="http://Scratch.ABC" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="Root">
<xs:complexType>
<xs:sequence>
<xs:element minOccurs="1" maxOccurs="1" name="AorBorC" type="AorBorCType" />
<xs:element minOccurs="1" maxOccurs="1" name="ABCOrMore" type="AorBorCMoreType" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:complexType name="AorBorCType">
<xs:choice minOccurs="1" maxOccurs="1">
<xs:element minOccurs="0" maxOccurs="1" name="a" type="xs:string" />
<xs:element minOccurs="0" maxOccurs="1" name="b" type="xs:string" />
<xs:element minOccurs="0" maxOccurs="1" name="c" type="xs:string" />
</xs:choice>
</xs:complexType>
<xs:complexType name="AorBorCMoreType">
<xs:choice minOccurs="1" maxOccurs="unbounded">
<xs:element minOccurs="0" maxOccurs="1" name="a" type="xs:string" />
<xs:element minOccurs="0" maxOccurs="1" name="b" type="xs:string" />
<xs:element minOccurs="0" maxOccurs="1" name="c" type="xs:string" />
</xs:choice>
</xs:complexType>
</xs:schema>
編集
質問が明確になったので、このスキーマは法案に適合するはずです
<?xml version="1.0" encoding="utf-16"?>
<xs:schema xmlns="http://Scratch.ABC" xmlns:b="http://schemas.microsoft.com/BizTalk/2003" targetNamespace="http://Scratch.ABC" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="Root">
<xs:complexType>
<xs:sequence>
<xs:element name="AorBorC" type="AorBorCType" />
<xs:element name="AorBorCMore" type="AorBorCMoreType" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:complexType name="AorBorCType">
<xs:choice>
<xs:element minOccurs="0" maxOccurs="1" name="a" type="xs:string" />
<xs:element minOccurs="0" maxOccurs="1" name="b" type="xs:string" />
<xs:element minOccurs="0" maxOccurs="1" name="c" type="xs:string" />
</xs:choice>
</xs:complexType>
<xs:complexType name="AorBorCMoreType">
<xs:choice>
<xs:element minOccurs="0" maxOccurs="unbounded" name="a" type="xs:string" />
<xs:element minOccurs="0" maxOccurs="unbounded" name="b" type="xs:string" />
<xs:element minOccurs="0" maxOccurs="unbounded" name="c" type="xs:string" />
</xs:choice>
</xs:complexType>
</xs:schema>