What does xsi mean when defining arguments

Question

I've seen for a while now that inside the arguments tag in di.xml or in the layout files the attribute xsi:type is used.
I took it for granted and used it like that, but I don't understand why the need for the xsi namespace. What does it mean?
The name attribute does not have a namespace.

Here is a random example.

 <arguments>
    <argument name="configurationsMap" xsi:type="array">
        <item name="default_message_identifier" xsi:type="array">
            <item name="renderer" xsi:type="const">\Magento\Framework\View\Element\Message\Renderer\EscapeRenderer::CODE</item>
        </item>
    </argument>
</arguments>

I know what the value of the xsi:type attribute means, but not why the need for the namespace.
This is not impeding my work, but it bugs me a lot.

Answer 1

Magento dev team wants to use the attribute named type which is deferred from XML Shema Interface. That's why they use xsi:type="blablabla", not type="blablabla". I don't find they declare an attribute named type. So, if you change the xsi:type="blablabla" to type="blablabla", the XML validation will throws an error.

Here is an example of default catalog_product_view.xml XML: catalog_product_view.xml

I have reconstructed the XML Schema Definition based on xsi:noNamespaceSchemaLocation path of the XML itself: page_configuration.xsd

You won't see any definition of type attribute in element named argument. Therefore, if you use <argument type="string">, the XML becomes invalid.

You may try the validation result of those XML and XSD here: W3C XML Schema (XSD) Validation online. Try to change the thing you asked, let's say, <argument xsi:type="string"> to <argument type="string"> and validate the XML, you'll see an error.