With Nexus Series Cisco had started a new regime for license driven approach instead of software (IOS) driven approach. Prior to this Cisco use to push a higher or a different IOS for all kind of feature upgrades. Licensing enables specified features...
http://www.ebrahma.com/2013/12/understanding-cisco-nexus-series-licences/Labels: cisco-nexus-licenses-details, cisco-nexus-licensing, NX-OS-licenses, understanding-NX-OS-licensing