Rule 18 covers "The Internet, Electronic Media, and Other Nonprint Sources". Examples of these sources include Web sites, commercial databases, microform, films, and video and audio recordings.
According to Rule 18.2, The Bluebook prefers citing to traditional print sources "when available, unless there is a digital copy of the source available that is authenticated, official, or an exact copy of the printed source." (Rule 18.2) Rule 18.2.1 further defines "authenticated", "official", and "exact" copies.
Several Bluebook rules address how to cite to online formats of specific materials. For instance: