You do not have to use the abbreviation sections in a Journals term list, but if you do, keep these points in mind:
Enter variations of the same journal name together for one term entry.
Enter a consistent abbreviation format in each row. For example, all Index Medicus abbreviations could be entered next to Abbreviation 1, another format of abbreviated names next to Abbreviation 2, and so on.
Only the first term for each journal (the one in the Full Journal slot) is transferred when you insert a term.
Use the Journal Abbreviation option in the styles to pick which form of the journal names should be used in the bibliography (see Creating Bibliographies with Journal Abbreviations).
EndNote can remove periods from abbreviations during formatting. EndNote cannot add periods to an abbreviated journal name.