In the case of older films, musical numbers may be missing due to rights problems. Often TV stations themselves will cut out portions of films that are not politically correct for their viewing public. Some labs will separate the sound from the image and make a fresh track. Units refer to the original number of reels a 16mm print was issued on. Units can range from 10 minutes to 40 minutes.
Reels are the plastic or metal reels the film is physically transferred to for projection.