Firstly, cutoff means the marks of the last person in that particular group.
So let’s say BITS Pilani has only 50 seats for CSE and lets say 100 people apply for that branch. The marks of the 50th person who got CSE will be called the cutoffs for that branch.
The same goes for any other branch. The marks of the Nth person who got that particular branch where N is the number of seats available for that branch.
Therefore for example if the first 10 people who got CSE(based on their rank) decide to not take admission, The next 50 people would get CSE and hence the cutoffs would decrease