The A1 is Mediator listens on the northbound interface in of the RIC for policy guidance. The northbound caller (e.g., non RT RIC, RIC Dashboard, SMO, etc.) creates policy types and 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
This wiki page may be useful for discussion about the A1 Mediator, feel free to post comments here.