how do journal name abbreviations work?

I want to make sure journal  name abbreviations are correct as listed in PubMed. Can’t find much info about how they work. For example, Archives Gen Psych.ens output style defaults to “use abbreviation 2” for journal name.  That shows the full journal name for my citations–but maybe  because I entered full journal names. PubMed seems to download them in my library in abbreviated form. What/where is the abbreviation 2, and how should I enter citations to make sure they seamlessly switch from full journal name to abbreviated journal name depending on the journal’s output style requirements?

Thanks 

See the information on our website at:

http://www.library.uq.edu.au/faqs/endnote/journal_terms.html