All RAC/WL tickets have 2 ’numbers’, the first is the position you join the waitlist, the 2nd is your current waitlist position.
So lets imagine you go to buy a ticket online (or at a reservation office), you find the perfect train, select your chosen class, enter your date of journey, then notice that the ticket offered is WL 10/WL 4, you buy the ticket, so we’ll use this as an example. (with 4 RAC places)
WL 10/WL 4 means that you join the waitlist at position 10, but due to cancellations (before...
more... you bought the ticket) you have already moved to waitlist position 4, the first number will not change, if there were another 3 cancellations, your new waitlist position will be WL 10/WL 1, another 3 cancellations would take you to WL 10/RAC 2, a further 2 cancellations would take you to WL 10/CNF (confirmed reservation, though you wouldn’t find out your carriage and berth number till a couple of hours before the train departs)
Think of standing in a queue, as you go to join the queue you are given a number indicating the position you join the queue (eg. WL 10), this number will remain the same as that IS the position you joined the queue, all those in front of you have either confirmed, RAC or better waitlist positions than you, if any of the people in front of you decide to leave the queue, you move forward, and closer to a confirmed seat/berth, some in front of you may have already left the queue before you joined, so if 6 people had already left the queue, you’d be joining the queue in position WL 10, but you’d already be at position WL 4.