Final answer:
In the early stages of war mobilization, Category I retirees provide experienced personnel, due to their recent active duty service and current knowledge.
Step-by-step explanation:
During the first stages of mobilization in time of war, experienced personnel are provided by Category I retirees. These are retirees who have recently left active duty and are still considered key to the operation and function of military units. They are often among the first to be recalled to active duty, should the need arise, because of their up-to-date experience and knowledge of current military practices and procedures.