Moving some information into a box or sidebar creates room in the main body text. For example, instead of cluttering up a profile with details of the resume, you can put it into a sidebar and devote the main bar to capturing the person. Some types of information, such as lists, can be easier to read in a box than in a paragraph.
Boxes allow readers who want more information to get it when they want it and return to the main text. For instance, you can put the full text of a document in the box and quote from it in the main bar. A printed box is the equivalent of a link in online, although linking has no space limitations.
Most writers and editors assume that readers read the main text first and boxes later; why else would we call them side-bars, as in “beside” or “aside?” Actually, readers are just as likely to read the box first because it’s shorter. So the contents of the box must make sense if read first. The box needs a good headline that tells the readers what the box is about.
Some people read only the box, so it must stand alone, make sense without the main bar. Readers should not be required to read both parts to understand either part. Readers tend to sample the box and not read it straight through. The secret is to keep boxes simple, perhaps using more than one.
Since they may be read first, charts, graphs, and graphics must make sense without reading the body text. A photo and its caption will be read first, and must not depend on the main text. Careful labeling solves these difficult problems of clarity.
You can put signals in the main text that point to the box or to other apparatus, like this: “(See box).” You can duplicate some of the information in the main text and the box so each makes sense.
Many publications assume that editors will break out boxes from the writer’s submitted text. But you want to do it yourself to make sure it comes out right. If you don’t know how, ask friendly editors to teach you. It’ll save them time later.