in sleeper class after filling of full seats/berths
an RAC ticket will be issued which means reservation against cancellation, in this u will get seat but not berth
and after RAC waiting list tickets are issued
so ur WL ticket should move to RAC and then confirmation
but...
more... for every train according to number of seats there will be a scope for ticket getting confirm till few WL
so just tell ur PNR for prediction in ur train, class
regards