Every QCI is associated with a Priority level.
Priority level 0.5 is the highest priority assigned to "Mission Critical delay sensitive signalling".This is assigned to QCI 69.
QCI 5 has priority 1 which is used for IMS signalling.
QCI 1 has priority 2 which is used for VoLTE.
The main idea of having the priority levels is to discard the traffic associated with the lowest priority level traffic when there is a congestion.