The A1 is Mediator listens on the northbound interface in of the RIC for policy guidance. The northbound caller (e.g., non RT RIC, SMO, etc.) creates policy types and policy instances through A1, and subsequently A1 exchanges messages are exchanged with xApps via RMR.
Please see here for the docs: https://docs.o-ran-sc.org/projects/o-ran-sc-ric-plt-a1/en/latest/index.html
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