In cameras from different manufacturers, the film plane varies in distance from the lens.
Thus each lens used has to be chosen carefully to assure that the image is focused on the exact place where the individual frame of film or digital sensor is positioned during exposure.
It is sometimes marked on a camera body with the 'Φ' symbol where the vertical bar represents the exact location.
Cameras may bend the film stock or even plate stock slightly to compensate, improving the area of critical focus and sharpness.
Nevertheless, the general concept of a focal plane is understood to refer to this position in the camera sensor relative to the lens.