HI , let me explain in high level .... In LTE RRC is responsible to configure all UEs for their layer configurations.
For suppose 4 UES are try to attach with enodeb, for this 4 UES some common configurations and some dedicated configurations are needed. common configuarations for all the uses will be provided by broadcast SIB2 information.
Now for each UE its dedicated information will be configured while RRC connection setup or Reconfiguration.
Now as per your query, MAC configuration is also part of RRC connection setup. In this message with dedicated info MAcMainConfig info gives the MAC level configurations.
Broadcast information is same for all UEs, but dedicated info may or may not same for all Ues.
Regards,
Suchi