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For the PIX 506E there is only one license that can be purchased which is the 3DES/AES and DES Encryption Licenses. Right now I think you can download that license for free. The following link will provide you with details on that license and the data sheet on that PIX (scroll down about half way for the license information).
The following link will give you the data sheet and describe the three different licenses available with upper models of the PIX as it relates to the 515E (restricted, unrestricted, and failover).