Specifying all Components of a Type

To retrieve all components of a particular type, use the wildcard symbol (*). For example, to retrieve all custom objects:

1<?xml version="1.0" encoding="UTF-8"?>
2<Package xmlns="http://soap.sforce.com/2006/04/metadata">
3    <types>
4        <members>*</members>
5        <name>CustomObject</name>
6    </types>
7    <version>48.0</version>
8</Package>

The wildcard symbol does not apply to all metadata types. For example, using the wildcard with the CustomObject type name will not retrieve standard objects. To retrieve a standard object, you must explicitly name the object in package.xml. Likewise, if you want to retrieve custom fields defined on standard objects, you must name the object and field.