How to get rid of DTD/Schema alert?

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Problem

When opening a file, you will sometimes face an error message indicating that certain attributes of certain items are not defined in the schema.

DTD/Schema alert in XML, how to solve it effectively.

Steps to follow

First of all, you must opt for "Schema Upgrade Utility"that you can find there:

File> Utilities> Data Utilities> Schema Upgrade Utility

Guide to removing the DTD/Schema alert in your XML editor.

Then, when this window appears, click Upgrade.

DTD/Schema Alert: How to get rid of it effectively.

The problem is now solved.