Very interesting blogpost from Rob Zoeteweij, make sure to download his instructions (PDF) regarding the Auto Discovery feature of targets in Enterprise Manager 12c.
What’s cooking at Oracle – Rob Zoeteweij
Using Enterprise Manager 11 and earlier, you needed to install an Agent any time a new Server was introduced in your datacentre.
Then after you would have created a new Database or any other Target Type, you would need to perform a discovery using the Agent that was monitoring the specific server.
Starting with Enterprise Manager 12c your Targets can get discovered automatically even if no Agent has been installed yet.