India's VIP security - What are the 'Z+',' Z', 'Y' and 'X' categories?

The Indian government has removed the The Special Protection Group (SPG) security cover for the Gandhi family, following an intel report that it faced no immediate threat. The family will now get 'Z+' security by the CRPF on an all-India basis. The government had recently also revoked the SPG cover for the former Prime Minister Manmohan Singh. Here is a ready reckoner for the VIP security categories available in India.