Hey Prakhyat, spot rounds are special rounds after all normal rounds are done. Spot rounds are conducted by the college itself rather than the exam conducting competent authority. This has a few implications.

There is no fixed cutoff as such. Whoever with the lowest rank is present at the time, with all necessary documents and the fees, and opts for the said branch/course is given admission. This makes the spot round very unpredictable, but it also increases your chances of getting into the college.