The \bibitem command generates an entry labeled by label. If the label argument is missing, a number is generated as the label, using the enumi counter. The cite_key is any sequence of letters, numbers, and punctuation symbols not containing a comma. This command writes an entry on the aux file containing cite_key and the item's label. When this aux file is read by the \begin{document} command, the item's label is associated with cite_key, causing the reference to cite_key by a \cite command to produce the associated label.