Today's lenses are machine made by very precise computer controlled equipment and are very uniform and precise. Consequently, their manufactured focal length is very consistent and accurate. Back in the good old days, when lens elements were ground by hand by artists under the complete control of error prone human beings individual lenses did not normally have the exact same focal length as was engraved on the bezel. For example, a postwar 50mm f1.5 Sonnar would normally have an actual focal length which would vary between 48mm and 53mm. If one took the mathematical average of the actual focal lengths of all the 50mm f1.5 Sonnars produced by Zeiss in a year it would probably add up to very close to 50. The Bezel engraving was, at that time the average focal lengh of all lenses produced. This manufacturing tollerance may have some practical consequences, depending upon your photographic requirements.
The focusing helical threads cut into the focusing mount in the camera are designed for a lens having a focal length of 50mm. If the lens that is on the camera deviates from 50mm there is going to be a mis-match between the actual focusing point of the lens and the distance engraving that is indicated by the rangefinder. Since it is normal practice to set the lens focus at infinity, this mis-match will become greater the closer the object is to the camera. Usually, this is not noticable in the pictures the camera produces because this mis-match is usually much smaller than the rangefinder tollerance when added to the lens depth of field at normal picture taking typical distances.
It is when the camera is used at close distances for copying or for the purpose of "testing" the lens that this mis-match can become evident, particularly if the negatives are examined under high magnification, for example, in a microscope or if the negatives are used to produce large enlargements. In some cases, if your demands are extremely precise it may be necessary for you to test your lens at various distances, map the discontinuities, and then apply focusing correction factors in actual practice.
If you plan to use your camera for very precise closeup work it may be optimal to have the lens and body combination precisely focused at the distance you plan to work with most often. In this case, the deviation will increase when the object is moved away from this point, instead of from infinity.
It is my normal procedure to calibrate the "normal" focus point of each lens at infinity using a Leitz military precison collimator. If you intend to use your camera and lens for closeup work, and you find that the results are not suitable, it is likely that much better results can be obtained by having your normal lens calibrated at your working distance. If you intend to use your camera for both normal and closeup work, it would probably be best to obtain a lens to be used only for copying and have its focus set at the copying distance.
While I've mentioned only the 50mm f1.5 Sonnar as an example, these notes apply to all lenses made for the Contax 35mm line of cameras including the 50mm Sonnar and Tessar, 21 and 35mm Biogons, the 85 and 135mm Sonnar, and other lenses special and rare made by other manufacturers.