- Если с XML не имели дело, ознакомтесь с основами:

- [Что такое XML](https://habr.com/ru/articles/524288/)

- Дополнительно:

- [Attribute vs Element](http://stackoverflow.com/questions/33746/xml-attribute-vs-xml-element#33757)

- [Лекции по XML](http://genberm.narod.ru/xml/lections.html)

Схему можно сгенерировать в IDEA (в контекстном меню в xml файле -> _Generate XSD Schema from XML File..._) и поправить вручную

<?xml version="1.0" encoding="UTF-8"?>

<UsersWithMeals xmlns="http://javaops.ru"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://javaops.ru usersWithMeals.xsd">

<Users>

<User enabled="true" email="user@yandex.ru" caloriesPerDay="2005" name="User"

password="password" registered="2023-01-01T13:00:00" roles="user">

<Meals>

<Meal calories="500" dateTime="2023-01-30T10:00:00">Завтрак</Meal>

<Meal calories="1000" dateTime="2023-01-30T13:00:00">Обед</Meal>

<Meal calories="500" dateTime="2023-01-30T20:00:00">Ужин</Meal>

<Meal calories="100" dateTime="2023-01-31T00:00:00">Еда на граничное значение</Meal>

<Meal calories="500" dateTime="2023-01-31T10:00:00">Завтрак</Meal>

<Meal calories="1000" dateTime="2023-01-31T13:00:00">Обед</Meal>

<Meal calories="510" dateTime="2023-01-31T20:00:00">Ужин</Meal>

</Meals>

</User>

<User enabled="true" email="admin@gmail.com" caloriesPerDay="1900" name="Admin"

password="admin" registered="2023-01-01T13:00:00" roles="admin user">

<Meals>

<Meal calories="510" dateTime="2023-01-30T14:00:00">Админ ланч</Meal>

<Meal calories="730" dateTime="2023-01-30T21:00:00">Админ ужин</Meal>

</Meals>

</User>

<User enabled="false" email="guest@gmail.com" caloriesPerDay="2000" name="Guest"

password="guest" registered="2023-01-01T13:00:00">

</User>

</Users>

</UsersWithMeals>

<?xml version="1.0" encoding="UTF-8"?>

<xs:schema xmlns="http://javaops.ru"

xmlns:xs="http://www.w3.org/2001/XMLSchema"

targetNamespace="http://javaops.ru" elementFormDefault="qualified">

<xs:element name="UsersWithMeals">

<xs:complexType>

<xs:all>

<xs:element name="Users">

<xs:complexType>

<xs:sequence minOccurs="0" maxOccurs="unbounded">

<xs:element ref="User"/>

</xs:sequence>

</xs:complexType>

</xs:element>

</xs:all>

</xs:complexType>

</xs:element>

<xs:element name="User">

<xs:complexType>

<xs:sequence>

<xs:element ref="Meals" minOccurs="0"/>

</xs:sequence>

<xs:attribute type="xs:string" name="name" use="required"/>

<xs:attribute type="xs:string" name="email" use="required"/>

<xs:attribute type="xs:string" name="password" use="required"/>

<xs:attribute type="xs:int" name="caloriesPerDay" use="required"/>

<xs:attribute type="xs:dateTime" name="registered" use="required"/>

<xs:attribute type="xs:boolean" name="enabled" use="required"/>

<xs:attribute type="roles" name="roles"/>

</xs:complexType>

</xs:element>

<xs:simpleType name="roles">

<xs:list itemType="roleTypes"/>

</xs:simpleType>

<xs:simpleType name="roleTypes">

<xs:restriction base="xs:string">

<xs:enumeration value="admin"/>

<xs:enumeration value="user"/>

</xs:restriction>

</xs:simpleType>

<xs:element name="Meals">

<xs:complexType>

<xs:sequence minOccurs="0" maxOccurs="unbounded">

<xs:element ref="Meal"/>

</xs:sequence>

</xs:complexType>

</xs:element>

<xs:element name="Meal">

<xs:complexType>

<xs:simpleContent>

<xs:extension base="xs:string">

<xs:attribute type="xs:int" name="calories" use="required"/>

<xs:attribute type="xs:dateTime" name="dateTime" use="required"/>

<xs:attribute type="xs:boolean" name="excess"/>

</xs:extension>

</xs:simpleContent>

</xs:complexType>

</xs:element>

</xs:schema>