One of the strengths of RELAX NG is its flexibility in supporting a difficult but sometimes convenient concept called "ambiguous content models" in the SGML world, "non-deterministic content models" in XML DTDs, or the "Unique Particle Attribution rule" and the "Consistent Declaration rule" in W3C XML Schema.

Before you read any further into this chapter, let's be very clear 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 which create problems. We will see in this chapter how to use RELAX NG in ways which avoid these problems, making RELAX NG "type assignment friendly".