Footnotes (e.g. for codes or abbreviations) in Characteristics of studies tables can be added in either Full Text or Default view.
In Full Text view, select Characteristics of studies on the left-hand side menu. In each Characteristics of studies table (Included, Excluded, Ongoing, Awaiting Classification) there is a 'Footnotes (shared)' text field for every study, where you can add your footnotes. The footnotes will appear below every study in the table.
In Default view, select to edit an individual study and add footnotes for abbreviations. The footnotes will appear below every study of the same type, e.g. an abbreviation footnote added to an excluded study, will appear below every study in the Characteristics of Excluded studies table.