The file mnemonic.bib contains a list of mnemonics which may
be used in the `journal' fields of the entries in the bibliographic
database.
If you want to use the mnemonics, you should include mnemonic.bib
into the \bibliography call:
If you have an entry: `journal = aa' (note: without quotes or braces around `aa'), then BibTeX will cause the mnemonic to be replaced by the text Astron. Astrophys. in the final list of references.\bibliography{mnemonic,other bibliographic database files}
The list in appendix D contains the same journals and periodicals as section 001 `Periodicals' in Astronomy and Astrophysics Abstracts, Vol. 49A [Burkhardt et al.1990]. The abbreviations follow the recommendations of the ``International List of Periodical Title Word Abbreviations'', so you are strongly encouraged to use them.
If you want to cite periodicals which are not in mnemonic.bib, or if you think you will never use 90% of the mnemonics in the supplied file, you could create your own mnemonics file, and copy from mnemonic.bib the abbreviations which you do use.