By default, the citation labels are printed one after another, separated by commas and enclosed in brackets, using the main text font.
Some formats require other styles, such as superscripted labels.
You can accomodate such formats by redefining the following macros. By default, it produces a comma followed by a space. Otherwise, it should print the note. By default, the note is preceded with a comma and a space. Here is an example, showing you could produce citations as superscripted labels, with the optional notes in parentheses.