India has 3 different reservation categories-- Scheduled Tribe, Scheduled Castes & Other Backward Castes. These broadly refer to these three subsections - Tribals, Dalits & Others respectively. As per Supreme Court ruling, the total percentage cannot be more than 50%. No religion based reservation is possible within this framework ie reservation for Christians, Muslims etc.( But sub-groups can be accommodated under OBC)
Originally these quotas were for 20 yrs after independence & only for education & jobs in govt sector. However, they have been extended for blocks of years repeatedly & extended to promotions in jobs too. This has created a problem in that some reserved category particularly in OBC, who were not very backward to begin with, got high social mobility & took away most of the benefits. To curb this, creamy layer concept was used which specifies an upper family income limit beyond which OBC individuals cannot avail reservation benefits.
Now are these castes selected? It can be done both at Central & State govt. The major criteria is social &/or economic backwardness. The first two segments are more or less straightforward but problem arises in the third segment OBC.
Given the relaxed qualifications and the rapid rise in jobs possible, it attracts attention and many groups wish to associate within this OBC framework. Hence the conflict. In this case Patels in Gujara, Meenas in Rajasthan and Jats in Haryana.