The final reservation chart for a train is prepared typically couple of hours before its departure. In case of early morning or late night trains, this may be prepared much earlier during the previous night itself. The significance of this status to a reservation status is because of the fact that your reservation status could change any time until chart is prepared. Though very rare, it does happen that an RAC ticket go back to wait-listed status. It is also possible as per Railways that a ticket that moved from WL to CNF can move back to RAC or WL. This will not happen for a ticket which was CNF during booking itself.