"Below the line" and "above the line" are film industry terms used to divide both a production team and its expenses. Above the line budgets cover the expenses of securing the rights to the narrative and the key creative personnel who will drive the creative vision for the project. People above the line include directors, producers, writers and most actors. They form the creative nucleus around which the project proceeds. The leader of the above the line crew is the leader of the entire crew, the executive producer.
Below the line expenses and personnel are those which serve to physically produce the film or television episode. Most of the people who work behind the scenes — make-up designers, production designers, grips, gaffers, sound designers, visual effects designers, extras and many, many more —are "below the line crew". The leader of the below the line crew is, on many productions, the line producer. However, in Doctor Who history, line producers haven't often featured, outside of the Steven Moffat era. At least on the majority of BBC Wales DWU episodes, the de facto below the line head has been the production manager.
During the original run of Doctor Who, the production structure was very different to 21st century Doctor Who. Most below the line talent belonged to a department within the British Broadcasting Corporation and were not specifically employed by the Doctor Who production office. The salaries of people like these were paid by the BBC, not by Doctor Who. This makes the lines of authority harder to see on so-called "classic" era serials. The functions of the modern-day line producer lay somewhere between the producer and the production unit manager.