One can check the seat availability for any train in Hotfoot by entering the train code or name between the station. Hotfoot Apps display seat availability calendar where one can see availability and fare for all classes and for all days in a single view. The availability status is also saved for popular routes so that passenger can see it without fetching again and again.
RAC stands for Reservation Against Cancellation. Under RAC a passenger is given a seat i.e. a half side-lower berth. It can get confirmed if passengers having confirmed reservation cancel their berth or seat. RAC tickets are not issued for short distance all-seating trains which have only chair car compartments like shatabdi, jan-shatabdi and double-decker. Also, no RAC tickets are issued under tatkal quota.
Regret (or no room) means that booking has been closed for that train for your dates due to very high number of waitlist tickets issued. Railway does not want to issue another waitlist ticket in such case because it thinks that there is no chance such high waitlist ticket will get confirmed. Therefore, one cannot book tickets if availability status is Regret.
Yes, the availability might change from Available to RAC or Waiting list if more and more people book ticket for a particular journey. Alternatively, it can also move from waiting list to available if confirmed passengers cancel their ticket.
This means that all the seats have been filled and if you still book ticket you will be in a waiting list of 100 people (General waiting list in this case). The initial number 150 would have been your actual waiting list but 50 (150-100) people have already cancelled their ticket. Your ticket will get confirmed only if 100 passengers booked before you happen to cancel their ticket. Waiting list can be of other types like RLWL,PQWL,TQWL etc.