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:

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.