OK! Found the answer! I was to much thinking of the SOAP messages, when this is for HTTP Get and POST where is a string element.
E.g.
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HTTP/1.1 200 OK
Content-Type: text/xml; charset=utf-8
Content-Length: length
<?xml version="1.0" encoding="utf-8"?>
<string xmlns="http://www.home.NET">string</string>
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And if there is "<s:element name="string" nillable="true" type="s:string"/>" in the schema it means that "if {nillable} is true, then an element may also be ·valid· if it carries the namespace qualified attribute with [local name] nil from namespace
http://www.w3.org/2001/XMLSchema-instance and value true (see xsi:nil (§2.6.2)) even if it has no text or element content despite a {content type} which would otherwise require content. Formal details of element ·validation· are described in Element Locally Valid (Element) (§3.3.4)." (W3C XML Schema Part 1: Structures Second Edition)