Question

what is the criteria used by the census to classify the migrants

Answer 1

Census take place every 10 years and the aim is to collect as much information as possible on the population of the country, it's people, tribes, religions etc. All this information helps the government to make better decisions and develop national policies.
In order to define a migrant, Census, specifically ask about the 'Place of Birth' or 'Domicile' as mentioned on the Identity card of a person.
Of course what categorizes as Migrants also differs, e.g. out of city, out of State, international etc.

Answer 2

Answer:

1. Place of birth, 2.place of residence