The default value of minOccurs and maxOccurs is 1. So if you don't specify a value the element should be exactly present once. Now in your example you have set the minOccurs value to 0, but you haven't set the maxOccurs value which will have the default value. This means that the maxOccurs is 1. Now this element could be present 0 or 1 times. In your example this element is present 1 time which is within the valid range.