The A1 Mediator listens on the northbound interface of the RIC for policy guidance. The caller (e.g., non RT RIC, SMO, etc.) creates policy types and policy instances through A1, and subsequently A1 exchanges messages 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.