The NX DMX allows you to add 2 extra physical DMX outputs to any ONYX system. The box itself doesn't hold any additional licenses for extra universes, it only provides the physical 5 pin outputs. The box can be configured to provide two extra outputs or two extra inputs or any combination of Input/Output, and also provides RDM functionality on it's outputs.
NX DMX Configuration
When attached to an ONYX system, the NX DMX will default to outputting DMX universes 1 & 2. This can be configured via the USB2DMX page in the System Settings.