What does xmlns="" exactly mean

Question

Given the following (piece of) a soap call;

<m1:NextCommencementDateInput xmlns:m1="http://foo.bar.com/Types"&gt;
    <aDate xmlns="">2010-06-02</aDate>
</m1:NextCommencementDateInput>

Apperantly this is the same as (when validating against the xsd using XMLSpy)

<m1:NextCommencementDateInput xmlns:m1="http://foo.bar.com/Types"&gt;
    <aDate>2010-06-02</aDate>
</m1:NextCommencementDateInput>

So what does xmlns="" do exactly ?

Edit: To elaborate why I'm asking this is because I'm calling a third party and they are now stating that we should remove xmlns="" from our requests. I however think they are the same and they should change their side.

Answer 1

According to the XML Namespace specification (§6.2), they are completely identical other than for the extra attribute itself (which your implementation may or may not hide from you).

Answer 2

xmlns="" clears definition of default namespace (aka empty prefix). After this declaration all elements without prefix are considered to have null namespace.

So the difference is:

• First example (with xmlns="") clears empty prefix so aDate element has null namespace.

• Second example doesn't clear it. Namespace of aDate element depends on namespace declaration in containing scope. If there is active xmlns="some:namespace" declaration, aDate will have this namespace. Otherwise it will have null namespace.

Additionally some XML parsers complain on xmlns="" if there is no active xmlns="some:namespace" declaration to clear...