Hello Veipunii Lana,

Here's how choice filling works. The choices which are on top of your list, if you are allotted any of those seats, you will get that seat and the rest choices which lie below that shall get nullified.

That means, you should be careful in order to number your choices in the choice filling process. Hope that helps 

Hi

Well, in regard with seat filling and locking in JOSSA counselling for JEE Advanced, you fill in the colleges along with their respective courses in order of your preference. If your rank is sufficient to get admission in a filled choice, you are alloted that seat, otherwise the next preference is checked upon. It does affect your cjhances of getting a seat as the filling depends upon the number of candidates filling that choice , who are above your rank. Preferences of seat filling would be given to the person having better rank. So, if more candidates fill a particular choice you have chances into and have better ranks, your admission chances  in that particular choice become low.

Hope this helps!!!