hi,
In Symantec Endpoint Encryption you don't apply .slf file, it's rather a paper license. The reason for reinstalling is the End User License Agreement (EULA) you accepted installing the trial version, where it says this is for evaluation purpose only - not production.
To answer your second question, when the disk is encrypted and you authenticate to it, the decryption key is loaded so the access to the data is available. When you logoff, decryption key is not anymore available so your data is encrypted.