It isn't a header, it's a schema reference. The schema reference points to a schema definition so that XML processors can automatically validate the XML.
The big difference here is that you're moving from a DTD to an XML schema (sometimes known as "xsd"). DTD's are the older of the 2 schema definition types and they are more limited, so newer specs use xsd's instead.
Whether you use a DTD or a schema, you do need to use the version that matches the XML content you're defining. Otherwise the XML parser will completely reject your XML without passing it on to the code that actually employs the XML.
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