This is recently introduced feature and hence not showed in all the PNR.
The logic is simple. It takes the reading from the 120th Day and shows here. If you see ">" symbol that means the reading was taken after 8AM so some seats might have been booked.
As regards to the Seats, then it displays from the Stations mentioned in the PNR. However, these are usually true from that Quota. For example, any train starting from Bombay CST will have same quota/seats till Kalyan (KYN).