There are two categories of 'premium' seating. The first kind costs Rp. 75,000 (or equivalent in RM or baht) and allows a choice of seating in row and the emergency exit rows.
The second kind allows passengers to select a seat in row 2-6 and cost Rp. 15,000 (or equivalent in RM or Bath).
If you don't select either option you get allocated a seat elsewhere in the aircraft.
I don't really see the point of paying for either option, although if I was doing KUL-India/China I might want to cough up because those flights are 4 hours ish.