NPE is no payload encryption -- No use for IPSEC surity etc
1. Universal images with the "universalk9" designation in the image name: This universal image offers all the Cisco IOS features including strong payload cryptography features such as IPSec VPN, SSL VPN, and Secure Unified Communications.
2. Universal images with the universalk9_npe" designation in the image name: The strong enforcement of encryption capabilities provided by Cisco Software Activation satisfies requirements for the export of encryption capabilities. However, some countries have import requirements that require that the platform does not support any strong crypto functionality such as payload cryptography. To satisfy the import requirements of those countries, the `npe' universal image does not support any strong payload encryption. This image supports security features like Zone-Based Firewall, Intrusion Prevention through SECNPE-K9 license.
If you look the link you will also see notes regarding Maintenance release M, M1 etc