It is worth remembering that the optical power of the lens is measured in diopters. When the lens moves out of focus, an additional lens is required to change the focal length of the entire system. This is what kind of lens we need and used to call "vision". If you look closely, it will become clear that such a value does not reflect how well or poorly a person sees, this value only works for myopia or hyperopia.
The unit of vision itself is 100% vision. Its deterioration can be expressed as 0.9 0.8 ... etc. And what you mean by points -1 and +1 are diopters, i.e. a description of the refraction of light in the lenses of these glasses, originally intended not for patients in any way, but for specialists in optics who make these lenses; as well as for pharmacists who sell contactless lenses.