What is Relay and why am I seeing it installed in the Splice app?

Relay can send metering data to and receive information from select Inter Plug-in Communication (“IPC”) compatible iZotope plug-ins, including: Insight 2, Tonal Balance Control, Neutron 2, Neutron 2 EQ plug-in, and Neutron 2 Visual Mixer and Nectar 3.

It is automatically included for free when you install Nectar 3. You update it directly in the Splice application, just like all other plugins on the Rent to Own program.