This is the XSD describing the Text Block XML. It can also be obtained by running tbmigrate.exe with the /xsd flag. It will then be written in the file TBimport.xsd.
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="http://www.aia-itp.com/Repository/3.1/TextBlockImport" elementFormDefault="qualified" xmlns="http://www.aia-itp.com/Repository/3.1/TextBlockImport" xmlns:mstns="http://www.aia-itp.com/Repository/3.1/TextBlockImport" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="tbk">
<xs:complexType>
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:element name="par" type="Paragraph" />
<xs:element name="lst" type="List" />
</xs:choice>
<xs:attribute name="xsv" use="required" fixed="2.0.1" />
</xs:complexType>
</xs:element>
<xs:complexType name="List">
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:element name="par" type="Paragraph" />
<xs:element name="lst" type="List" />
</xs:choice>
<xs:attribute name="style" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="unordered" />
<xs:enumeration value="ordered" />
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
<xs:complexType name="Paragraph">
<xs:choice maxOccurs="unbounded">
<xs:element name="txt" type="Text" minOccurs="0" />
<xs:element name="fld" type="Field" minOccurs="0" />
<xs:element name="chr" type="Character" minOccurs="0" fixed="" />
</xs:choice>
<xs:attribute name="indentation" use="required" type="xs:nonNegativeInteger" />
<xs:attribute name="hanging-indentation" use="optional" type="Boolean" fixed="false" />
<!-- hanging-indentation is for internal use only -->
<xs:attribute name="font" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="normal" />
<xs:enumeration value="header" />
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
<xs:complexType name="Text">
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attributeGroup ref="FontStyle" />
<xs:attribute name="id" use="optional" type="String254" fixed="" />
<!-- id is for internal use only -->
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:complexType name="Field">
<xs:simpleContent>
<xs:extension base="String254">
<xs:attributeGroup ref="FontStyle" />
<xs:attribute name="set" use="required" type="String254" />
<xs:attribute name="id" use="optional" type="String254" fixed="" />
<!-- id is for internal use only -->
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:complexType name="Character">
<!-- special/whitespace characters -->
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attributeGroup ref="FontStyle" />
<xs:attribute name="type" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="LBR" />
<!-- Line break -->
<xs:enumeration value="NBSP" />
<!-- Non-breakable space -->
<xs:enumeration value="NBHH" />
<!-- Non-breakable hyphen -->
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="id" use="optional" type="String254" fixed="" />
<!-- id is for internal use only -->
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:attributeGroup name="FontStyle">
<xs:attribute name="underline" use="optional" type="Boolean" default="false" />
<xs:attribute name="italic" use="optional" type="Boolean" default="false" />
<xs:attribute name="bold" use="optional" type="Boolean" default="false" />
</xs:attributeGroup>
<xs:simpleType name="String254">
<xs:restriction base="xs:string">
<xs:maxLength value="254" />
</xs:restriction>
</xs:simpleType>
<xs:simpleType name="Boolean">
<xs:restriction base="xs:string">
<xs:enumeration value="false" />
<xs:enumeration value="true" />
</xs:restriction>
</xs:simpleType>
</xs:schema>