This is my understanding although I am not 100% sure of this. When there are a no. of trains for the between same set of origin and destination stations, some times, instead of indivial wait list for each of the trains, a Pooled Quota Wait List is created, where all the vacant seats are submitted to the pook and then these seats are alloted to the passengers in the PQWL. Any one can get a confirmed seat in any train depending upon the cancellations in trains. The chances of getting a confirmed seat increase with the PQWL.