Additional Secretary (often abbreviated as AS, GoI or Union Additional Secretary or Additional Secretary to Union of India) is a post and a rank under the Central Staffing Scheme of the Government of India. The authority for creation of this post solely rests with  Cabinet of India. 
Additional secretary is an
ex-cadre post, that means anyone can occupy it, but the position holder is mostly a career civil servant, generally from the Indian Administrative Service,   and is a government official of high seniority. The civil servants who hold this rank are either from  All India Services (on deputation; on tenure, after empanelment) or Central Civil Services (Group A; on empanelment). All promotions and appointments to this rank and post are directly made by the Appointments Committee of the Cabinet.
In the functioning of
Government of India, an additional secretary is the administrative head of a wing in a department. Additional secretaries -- on deputation -- can senior positions at the  United Nations, like as India's permanent representative to UNESCO. 
The post is somewhat analogous to a
principal secretary in a state government. But because of seniority, office-bearers can hold senior positions like as that of additional chief secretary to a state government, and in Union Territories, they can hold the position of Chief Secretary of a UT government.
Additional secretaries in the
Union Government   is somewhat analogous to  lieutenant general (Level 15/HAG) and equivalent ranks  in  Indian Armed Forces, but the rank of lieutenant general is placed above on the order of precedence.    
Additional secretaries rank 25th on
Order of Precedence of India    
Sir Richard Tottenham,
ICS had once expressed "In my opinion there is, or should be, no distinction of function, but only of pay between a joint and an additional secretary. Additional and joint secretaries should not be either cheap secretaries or expensive deputy secretaries." 
Powers, responsibilities and postings
The organizational structure of a department of the Government of India.
Additional secretary is the overall in charge with the necessary measure of independent functioning and responsibility of the wing of the department allocated and entrusted to him.
An additional secretary in charge of administration also exercises all administrative powers as head of the department wing of the ministry/department.  
Additional secretaries and
joint secretaries are responsible for filing all affidavits and responses before the Supreme Court of India.  
Prime Minister of India is the final authority on posting and transfer of officers of additional secretary level. Additional secretaries report to their departmental  secretary and ministerial/departmental cabinet minister.
In the Union government, the members head department wings in the departments and ministries of Union Government.
Government nominated Board members in the
Central Public Sector Enterprises/ Public Sector Undertakings are either of the rank of additional secretary or joint secretary. 
According to the
Seventh Central Pay Commission of India, IAS officers hold 98 out of 107 positions of additional secretary in the Government of India.
Emolument, accommodation and perks
An Indian Diplomatic Passport and an Official Passport generally issued to Additional Secretary (GOI).
All additional secretaries to Government of India are eligible for a
diplomatic passport. They are allotted Type-V (D-II and D-I) and Type-VI (C-II) apartments in areas like New Moti Bagh across Delhi by the Ministry of Urban Development (Directorate of Estates). 
The salary and emolument in this rank is equivalent
to  principal secretary to a state government and lieutenant general (Level 15/HAG) and equivalent ranks in Indian Armed Forces.  
Additional Secretary, GoI monthly pay and allowances
Base Salary as per
7th Pay Commission (Per month)
Level on Pay Matrix
(Minimum Pay) to (US$3,100) (Maximum Pay)
Reforms and challenges
Non-IAS civil services have complained to Government of India because of lack of empanelment in the rank/post of additional secretary on numerous occasions.
    
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