SEP Definition downloads

Are SEP client downloads determined by the liveupdate policy (4 hours) or by the heartbeat interval (defaul 30 minutes)? How often does the Symantec server download defs from Symantec?

So I understand this correctly, Symantec will release definitions 3 times a day, and SEPM will tune in every 4 hours with the Symantec servers and pull down the definitions or do they do this real-time as definitions are released? After such, the clients download based on their 30 minute heartbeat interval?

Any clarification appreciated. Thanks.