The .bbl file uses a type of list environment (such as
the itemize enviroment), which has the form:
\begin{thebibliography}{xx}
\bibitem[label]{key}
...
\bibitem[label]{key}
\end{thebibliography}
The thebibliography environment has an argument which should be a string at least the length of the widest entry label in the source list. Instead of using the numbers generated by the environment as entry labels, you can specify your own by using an optional argument to bibitem, but remember to increase the length of the argument to the thebibliography command. Consider the following examples:
\begin{thebibliography}{1}
\bibitem{latexbook}
Leslie Lamport. {\em \LaTeX~User's Guide \& Reference Manual}.
Addison-Wesley Publishing Company, 1986.
\end{thebibliography}
\begin{thebibliography}{Name YY}
\bibitem[Lam 66]{latexbook}
Leslie Lamport. {\em \LaTeX~User's Guide \& Reference Manual}.
Addison-Wesley Publishing Company, 1986.
\end{thebibliography}
In the first example, the entry label is generated by the environment. Note
that this will fail if you have more than nine entries, because the
parameter to thebibliography command is not wide enough. The
second example shows the use of your own entry labels.