What is decentralised counselling in WBJEE?

After centralized e counseling, many seats in many colleges still remain vacant as many students don’t get their preferred college so they don’t join.
So all the colleges conduct another round of decentralized counselling where you’ve to go physically and appear for counselling. If you’ve a high enough rank you may get the seat you like.
All the government colleges conduct decentralized at the same time whereas other colleges conduct at different times.