The nominal control voltage for the relay is 12 VDC. However, between 5 - 30 VDC will work, applied to pin 4 relative to the ground pin 2 on the Deep SUNA V2. The voltage can be applied to the relay any time after external power is applied to the instrument for a recommended 100 milliseconds. Unless the relay is already switched on, there should be a very quiet (but audible) click when the relay connects power to the SUNA V2 electronics, and the instrument should enter its boot up cycle.
The purpose of the relay is to keep external power applied to the SUNA with very low quiescent current draw, so the typical use case involves the SUNA constantly powered. If the application includes the ability to switch power to the SUNA effectively then the relay feature isn't necessary. Regardless of how power cycling to the SUNA is controlled, there should be a power-off period of at least 30 seconds between power-on cycles to ensure that the capacitors that prevent a hard shutdown are allowed to completely discharge and allow the SUNA to boot up properly during the next power-on cycle.