As such standard does not define mapping between QCI and RLC Mode (AM/UM) for a bearer. It is upto eNodeB RRM/Application to choose best suitable mode to deliver the services delivered through a particular bearer.
While establishing a radio bearer MME send QoS parameters and those all used at eNodeB to derive the corresponding RLC mode.
For example: QCI - 1 is used for voice data traffic which requires better user experience compare to a service which is FTP based.
eNodeB can use RLC AM for ftp kind of services while for the QCI "1", RLC UM is preferable because of putting re-transmission mechanism for voice data packets can make the user experience worse.
