Hi Espana82,
the general differences are:
Symantec Encryption Management Server (SEMS) runs on a customized linux server. With that product you can encrypt drives, emails, fileshares, create encrypted archives (pgp zip).
Symantec Endpoint Encryption Management Server (SEEMS) runs on a Windows machine. With that you can encrypt drives (no emails).
More detailed documentation you can find here:
Symantec Encryption Desktop 10.3.2 and Symantec Encryption Management Server 3.3.2 Documentation
https://support.symantec.com/en_US/article.TECH214498.html
Endpoint Encryption 11.1.0 Documentation and Getting Started Guides
https://support.symantec.com/en_US/article.INFO3452.html
If you purchase licenses, you can use any of the product provided the number of installations doesn't exceed the number of licenses acquired.