All are correct, In simple terms
SEPM: Symantec endpoint protection manager is a console from which you can install clients , update policy, update virus definition. All Managed clients report to this SEPM, visible inside the console
Managed client: Any computer which reports to SEPM is managed client.. This client can be managed from SEPM like updates, policy, reports,
Unamanged: Any computer which DOES not reports to SEPM is unmanaged client.. you cannot control it from SEPM,