While I was writing for one of the presentations , thought of sharing  this with you .

SCCM Discovery Methods :

•Forest Discovery
•System Discovery
•User Discovery
•Group Discovery
•Network Discovery
•Heartbeat Discovery
Takeaway : Heartbeat Discovery method is enabled by default in SCCM . Rest All Other are Disabled .
•Forest Discovery
•This Discovery Method is Used to Discover Forests in SCCM . This Method was Not available in SCCM 2007 .
•Forest Discovery can be used to Discover all the forests with in the Organization’s Environment .
•As a result of Forest Discovery , a Single Primary Site can be used to Manage Multiple Sites .
•Log File to Trace Issues Related to SCCM Forest Discovery :
Forest Discovery
•System Discovery
•In order to get System Data from Active Directory to SCCM , System Discovery Method has to be enabled . Once enabled system data from Active Directory to SCCM Starts to flow .
•System Discovery is disabled by Default for a Fresh SCCM Installation .
•Log file to Trace Issues related to System Discovery : adsysdis.log
•SCCM User Discovery and System Discovery.
•Both the Discovery Methods are Disable by Default .
•Log Files to trace issues related to User and Group Discovery are :
•Adusrdis and adsgdis Respectively .
•Heart Beat Discovery
•This is the only Discovery Method that is Enabled by default when we Install SCCM Afresh .
•The Purpose of Heart Beat Discovery is to make Modifications to Already Identified Objects .
•Heart Beat Discovery modifies DDR’s
•A DDR is a data Discovery Record .