All India Quota doesnt have Defense Quota candidates and thats the reason you do not see Defense quota as an option but see other reservation categories. Defense quota is only available at State Quota and it is not a guarantee that all States have Defense Quota reservation. So you should contact the respective State Counselling authorities for further details. 

There are 17 to 20 seats for MBBS allocated by Ministry of Health and Family Welfare for defence quota seats-only wards of ex serviceman from navy, army and airforce and wards from service-5 category can apply. The seat matrix varies each year and you need to apply a separate application after the NEET results are out. The application will be available on Kendriya Sainik Board website.