Difference between Deputation (ISTC) and Absorption:

When an officer is appointed from outside the cadre or from outside the direct line of promotion for a limited period by the end of which he will have to revert to his parent cadre, he is known to be on deputation or on short-term contract. The term deputation is used when officers from Central Government Departments, State Government or Union Territories are considered for appointment.   Short-term contract is also in the nature of deputation and this method is followed when services of suitable officers belonging to Non-government Organizations, e.g. Universities, Recognized Research Institutions, Public Sector Undertakings etc., are required for appointment to Teaching, Research, Scientific or Technical posts.

When an officer working outside the cadre or outside the direct line of promotion is regularly absorbed in the post/grade, the post is treated as having been filled by the method of absorption.

In cases where the field of promotion consists of only one post, the method of recruitment by deputation (including short-term contract)/promotion is prescribed so that the departmental officer holding the feeder post is considered along with outsiders who have applied for appointment on deputation basis.  This method is known as the Composite Method. If the departmental candidate is selected for appointment to the post, the post is to be treated as having been filled by promotion; otherwise the post is filled by deputation/short-term contract for the prescribed period of deputation/ short-term contract at the end of which the departmental officer is again  afforded an opportunity to be considered for appointment to the post.