Because the PMP 450 Series radios are software-defined radios, you have the ability to upgrade system features as they become available. While minor enhancements may be available at no additional charge, major feature enhancements often require the purchase of a license key. One example of this is the PMP Interoperability License Key which allows your PMP 450 system to interoperate with PMP 100 or PMP 430 subscriber modules (when available).