When the British were in India, for effective adminisitrative purposes, they made lists of Indian communities, using the terms' castes' and 'tribes'. The term caste was used for Jats (sub-caste) and also for Varnas (main castes).
Tribes were those original inhabitants who did not accept the caste system and withdrew into the forests and mountains. These lists were used by the Indian governments to create lists of communities for affirmative action.
The upper castes were classified as forward castes and the remaining communities were classified as lower castes further sub-divided into three groups.
The first category is the Scheduled Castes which includes in it Dalits.
The second category is Scheduled Tribes conssiting of the tribal communities in the forests and mountains, largely in north and North-East India. They are also called 'Adivasi', which means aboriginals.
The third category is called Other Backward Classes or Backward Classes which includes Sudra sub-castes and also untouchables who converted to other religions.
When it comes to preferential treatment in affirmative action by the government, the first prirority goes to the Scheduled Castes, next to the Scheduled Castes and lstly to the Other Backward Castes.
The concept of Other Backward Castes and affirmatrive action for them largely took place following the findings of the Mandal Commission constituted to indentify and include the relevant backward castes for affirmative action.