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