This kind of question is usually best answered by the spec. In appendix B, you can find the following:
<!ELEMENT message-driven (description?, display-name?, small-icon?,
large-icon?, ejb-name, ejb-class, transaction-type,
message-selector?, acknowledge-mode?,
message-driven-destination?, env-entry*, ejb-ref*,
ejb-local-ref*, security-identity?, resource-ref*,
resource-env-ref*)>
After message-driven-destination, there is a question mark. This means that the element can appear 0 or 1 time as a child of the message-driven element.