The second unit is a principal photography unit which does not typically film scenes with dialogue, and only occasionally works with the main actors at all. Second units are usually charged with filming establishing shots, some kinds of close-up, and cutaways — basically, the shots that don't comprise the main action of a scene. Second units work at the same time as first units, though most often in different locations. For instance, the second unit on Daleks in Manhattan was actually in New York City while the first unit remained behind in Cardiff to film the bulk of the episode.
It might be assumed that second units have only been a part of Doctor Who history since the advent of more American approaches to filmmaking permeated production beginning with the 1996 tele-movie, And it is certainly true that the BBC Wales family of DWU programmes make extensive use of second units. However, second units were occasionally used on location filming of the 1963 version of Doctor Who. John Nathan-Turner, for instance, was well known for his uncredited efforts as a frequent second unit director. (DOC: Myth Makers: John Nathan-Turner) He was known to have directed second unit of a motorcycle scene in Delta and the Bannermen in which he infamously allowed Sylvester McCoy to appear on camera wearing his glasses. (DCOM, INFO: Delta and the Bannermen)