Ajit, 

Jossa counselling is totally dependent on your preference list. When counselling will start you have to fill your choices according to your preference list. You may/may not be allotted a seat in first round. Suppose you are allotted a seat in first round. Then you have to go to reporting centre and choose the option float. 

In the second  round you will be allotted the same seat or the seat which is given priority in your choice filling list and which is above the current seat.  

For eg you have 7 choices named as 1,2,3,4,5,6,7.

In first round you get the seat no.  4 .

Now in second round you will either get seat no. 4 or seat which is above 4(that is 1,2,3).

This process will further continue in next round. 

Remember to choose the float option if you want to participate for next round. 

And one thing more,  choice below 4( that is 5,6,7) will become useless ,null and void. 

As you have given case of NIT CALICUT if you get something else in round 4/5/6/7 you won't be able to get back NIT CALICUT. 

If you still have any queries you can ask in comment section. 

BTW Dont worry for counseling jossa will conduct two mock counseling round to clear everything.