When I was studying to be a paralegal, we learned about a legal move called a sidebar. A sidebar in legal terms is when the judge summons a lawyer to come to the judge's bench to talk about something that he doesn't want the jury or the other attorneys to hear.
A lawyer can also ask the judge if he can approach the bench to talk about something off record. The opposing attorney is allowed to be there.
In general terms, a sidebar is a time when some people at a meeting, step aside to talk about information that they don't wish to share with the group.
These definitions kind of fit in with the sidebars in writing or on the computer. They talk about extra information off to the side. One difference with the law definition, the information isn't shared with the larger group.