It depends on the venue. At some KalaSudha concerts you choose a reserved seat when you book, as at The Bhavan in London. At others, tickets are sold by seating zone, and at the rest seating is unreserved, so you pick any free seat when you arrive.
The ticket options on the event's booking page show which applies, so check them when you book. For unreserved seating, arrive at doors to get the closer rows. See when to arrive.
Have access needs? See who to talk to about access.