One of the strengths of RELAX NG is that it's flexible enough to support a difficult but sometimes convenient concept called:

Before you read any further into this chapter, realize that as far as simple validation is concerned, RELAX NG processes ambiguous schemas happily. That said, when type assignment or data binding is involved, schema ambiguity may have consequences that create problems. You will see in this chapter how to use RELAX NG in ways that avoid these problems, making RELAX NG "type assignment-friendly."