When you save a message definition file, the definitions that it contains are checked to ensure that they make sense and provide sufficient information about the message. This action is called validation.
For XML, these checks mostly concern the uniqueness and validity of XML names in global elements and attributes, and also for elements and attributes within complex types or groups. Since elements can be rendered as attributes, and attributes as elements, uniqueness tests will sometimes detect conflicts between objects that are not obviously related, such as an attribute and an element.
Tests other than these tend to be both simple and obvious so that, for example, the message set property First Day of Week has to be the name of a day in the week.