DOF is a computed distance range of acceptable sharpness based on focal length, aperture, subject distance, and the value for acceptable circle of confusion for a given format. The "acceptable" part is somewhat subjective, but the typical values used to compute DOF tables and scales are based on things like typical enlargement factors and viewing distances for the format and the average limits of human vision. There is no way to calculate DOF without making those assumptions.
It is also perfectly reasonable to deviate from those assumptions, based on one's personal standards for sharpness or planned enlargement factor. In other words, you may decide that you are planning to make a big print, so the value used as the acceptable circle of confusion should be smaller in that particular case. Most people don't actually make that computation, but just stop down one or two stops from the aperture recommended by the DOF scale on the lens or DOF table, which is the result you would get by computing the DOF with a smaller than typical value for acceptable circle of confusion for that format.
You may also decide to stop down for other reasons, like reducing falloff of illumination with a wide lens or reducing spherical aberration or field curvature, since DOF isn't the only component of sharpness.