Hello,
To Answer your Question, when SEP version 12.1 clients are deployed with Packages from SEPM, these packages are created with Latest definitions in it.
So as soon as SEP clients gets installed, the clients are installed with latest definitions.
Now, incase, these clients are Mobile machines, and then connect to the SEPM for definitions however, say incase, they are unable to fetch the entire downloads, then the next time it downloads it would download the same definitions again.
i.e; It would fetch those days definitions which have not been received by Clients.
Here are few Article for you:
Symantec Endpoint Protection 12.1: Installing the Manager for the first time and deploying clients
http://www.symantec.com/docs/TECH163580
Creating custom client installation packages in the Symantec Endpoint Protection Manager console version 12.1
http://www.symantec.com/docs/TECH165801
Upgrade clients to SEP 12.1 by Auto upgrade feature
https://www-secure.symantec.com/connect/articles/upgrade-clients-sep-121-auto-upgrade-feature
Also,
See About the types of threat protection that Symantec Endpoint Protection provides.
See Configuring client installation package features.
After installation, you can enable or disable the protection technologies in the security policies.
See About enabling and disabling protection.
See Performing tasks that are common to all security policies.
Hope that helps!!