Software Activation Feature Licenses:
These licenses are enforced through Cisco Software Licensing Framework.
The HSEC-K9 license removes the curtailment enforced by the U.S. government export restrictions on the encrypted tunnel count and encrypted throughput. HSEC-K9 is available only on the Cisco 2921, Cisco 2951, Cisco 3925, Cisco 3945, Cisco 3925E, and Cisco 3945E. With the HSEC-K9 license, the ISR G2 router can go over the curtailment limit of 225 tunnels maximum for IP Security (IPsec) and encrypted throughput of 85-Mbps unidirectional traffic in or out of the ISR G2 router, with a bidirectional total of 170 Mbps.
The Cisco 1941, 2901, and 2911 already have maximum encryption capacities within export limits. The HSEC license requires the universalk9 image and the SEC license pre-installed.

Product Activation Key (PAK)
A PAK is an 11 digit alphanumeric key created by Cisco manufacturing that identifies a specific software purchase. PAKs are not tied to a specific device until they are used to generate a software activation license. PAKs can be ordered independently for feature upgrades. PAKs do not have an expiration date and can be redeemed any time after purchase.
Delivery of PAKs can be paper based or electronic. Paper based PAKs begin with SL- for technology package licenses and FL- for Feature licenses. Customers ordering paper based PAKs are sent a software claim certificate by mail identifying their PAK string. Below is the example of a PAK.