The Norwood Scale is a classification system to measure the stages of male pattern baldness. Each stage is given a number, and the higher the number, the more evident the hair loss is. Stage 6-7 on the Norwood Scale is considered the most severe stage of male pattern baldness, and this can mean that men who are at this level might not be eligible to have a hair transplant.
When you have a hair transplant procedure, hair from a donor section of the scalp is transplanted into the balding areas that the patient wants to be covered. This means that there needs to be a sufficient amount of hair available for use in this process. With a stage 6-7, there is often insufficient hair, if any. And therefore a hair transplant cannot be carried out. However this does not mean that a hair transplant with this level of hair loss is always impossible. It mostly depends entirely on the patient, the specialist, and the potential donor area.