I suppose if people were defined as sidebars, that is what you would call Watson—a sidebar to Sherlock Holmes. He isn’t always necessary, but makes the story better for his presence. Likewise, a sidebar is a block of information that is generally run in a column next to the article and often set apart within a box. It contains information that compliments the article, but would not be appropriate within the body of the article itself. A sidebar might focus on statistics, additional resources on the topic, a list of affiliated organizations, quotes from others on the topic, a quiz on the topic, a glossary of terms used in the article, an anecdote or case study on the topic, historical background, differing points of view, etc. A sidebar is often the professional touch that will sell an article. Gives it that extra pizzazz.
Some publications use a lot of sidebars and either require them or consider them a plus. Other never or seldom use them. The market guide will indicate if they use them, and studying a few sample copies will tell you very quickly how many and what kind they use most often. Not sending a sidebar to an editor who prefers them is as bad as sending them to an editor who never uses them.
Every time you write an article, make a list of possible sidebars. Some articles will support more than one sidebar, so don’t limit yourself or the editor. In your query, offer more than one choice for sidebars.
Should you expect extra payment for a sidebar? It depends. If the sidebar is included in the total number of words you are being paid for—you promised and delivered a 2,000 word article, including sidebar—then No. If, however, you deliver a 2,000 word article without a sidebar, and they later ask for an additional sidebar, then, Yes, you should expect and get extra payment. The amount of payment will depend on whether you already have the additional information in your notes, or if you are going to have to do new research.
When submitting a sidebar, type them on a separate page (or pages) identified at the top as “Sidebar to (name of article);” or “Sidebar #1 to (name of article)” if you have more than one; or “Statistical Sidebar to (name of article)” or “Case-Study Sidebar to (name of article)” if you want to identify them more closely. Type each as a separate manuscript and use your common sense in identifying them.