The routing data (in Submit job and in communications jobs) is matched against subsystem Routing Data entries in the subsystem description. The first match allows the execution class found selects the program, execution class object and subsystem memory pool the job runs.
I found the following link to IBM's subsystem documentation that details subsystem management (including routing entries) at https://www.ibm.com/docs/en/i/7.1?topic=work-managing-subsystems
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