Hi Adensoul,
Was this SEP purchased in a bundle / security suite with other Symantec products? Maybe something like Symantec Protection Suite Advanced Business Edition (http://www.symantec.com/business/products/multimedia.jsp?pcid=pcat_security&pvid=prot_suite_abe_1)
If so, then the most likely case is that you will need to apply that .slf to Symantec Mail Security for MS Exchange and perhaps some other products, but SEP 11 will not need to have a license file manually applied in order to function.
Please let the forum community know if this resolves your question!
Thanks and best regards,
Mick