The phrase "reel change" refers to the process of replacing one reel of film with another when watching a movie. It is done when one reel ends and needs to be switched out for the next reel to continue the film. Full definition
Eco-CutTM enables 20 percent less film to be used per pack sealed, which gives longer running time between film reel changes and impressive cost savings. (fdbusiness.com)
(As «cigarette burns» are present at reel changes, this was apparently not sourced from the camera negative.) (filmfreakcentral.net)
Eighty years of wear and tear, including reel change markers, are quite evident. (dvdizzy.com)