Dear Aspirant

The authorities will reserve certain percentage of seats for sdifferent category candidates  like- OC, BC, BCM (BC Muslim), MBC & DNC, SC, SCA and ST.

SCA stands for Scheduled caste Arunthathiyars (SCA)

As there is not any SCA to SC counselling they are just categories.

you can also get further information by visiting below link-

https://engineering.careers360.com/articles/tnea-seat-reservation

For further query feel free to ask!

Hope it helps! GOOD LUCK!