Hello Marvin,
I beleive you are talking about PIX Failover licence, here's a quick explanation -
The failover licence is necessary if you wish to connect two PIXs together and perform either non-stateful or stateful failover. If you do not currently have a licence for failover, and wish to add it for your PIXs, it is a simple matter of paying Cisco the necessary money, and Cisco will then give you a key that you can use to unlock the failover feature.
A limited licence is typically indicated by the letter R in the software licence. A limited licence indicates that you have not purchased all the features for you PIX. However, if your licence is indicated by the letters UR, this indicates that you have an unrestricted licence and thus have access to ALL of the features of your PIX, such as, Encryption, Failover, and Connections.
Hope this helps -