As explained in the documentation of simulator, two modes are possible how to select carriers as you have mentioned:
CC_selector can be set to:
1. ‘all’ – Here all active users support Carrier Aggregation. For more details you can check the file +CC_selectors:all_CC_selector.m.
2. ‘cyclic’ – Since users have ID-s to be identified, user with ID =1 will be assigned to the first carrier component, user with ID = 2 will be
assigned to the second carrier component and this continues in a cyclic fashion. This assignment is done for users served by one eNodeB. For more details you can check the file +CC_selectors:cyclic_CC_selector.m.