One item in a bibliographic list.
The following, in order:
<label> Label (of an Equation, Figure, Reference, etc.), zero or one
One or more of any of:
Citation Elements
<citation-alternatives> Citation Alternatives
<element-citation> Element Citation
<mixed-citation> Mixed Citation
<nlm-citation> NLM Citation
<note> Note in a Reference List
<x> X - Generated Text and Punctuation