A return path email header can take two forms: 1. Plain email form: Return-path: <steve@tresys.com> 2. Empty form: Return-path: <> I've found 2 different ways to model this: #1 - with a choice <xsd:choice> <xsd:element name="Empty" type="xsd:string" dfdl:lengthKind="explicit" dfdl:length="0"/> <xsd:element name="PlainEmail" type="PlainEmail"/> </xsd:choice> #2 - with min/maxOccurs <xsd:element name="PlainEmail" type="PlainEmail" minOccurs="0" maxOccurs="1" dfdl:occursCountKind="implicit"/> When I'm faced with selecting one of these implementations over another, how do I think? What criteria should I be using? Is there another, better way that I haven't even thought of? TIA