ECR stands for Emigration Clearance Required and ECNR - Emigration Clearance Not Required. The clearance is from the Protectorate of Emigrants. Some category of people (graduates for one if I remember correctly) don't need clearance. The passport officer is the best to answer the question.
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This ECR/ECNR stamping is applicable only for visitors to non european countries like middle east and africa. It is also not required for visitors to U.S.. If you are a matriculate then you do not require for even Africa and Middle east. This is to ensure that you do not go to these countries as manual labour and get deported at India's expence.