Many citation styles require that journal titles are abbreviated (e.g. using a journal abbreviation list.
Zotero 4.0 and later can automatically abbreviate journal titles in Index Medicus/MEDLINE format. This option is available in the Document Preferences window. When it is disabled, Zotero uses the abbreviation from the “Journal Abbr.” field in the client.
A planned feature for Zotero is support for choosing among several abbreviation lists, so that different styles can use different journal abbreviations (e.g. Proceedings of the National Academy of Sciences can be abbreviated as either Proc. Natl. Acad. Sci. U. S. A. or PNAS).