The relay is mainly used for more general-purpose switching, such as garage doors. The firmware does not support the relay yet, and there is no documentation about it in the user manual. If you need it, you can modify the source code to add support for it (the digital pin that controls the relay is 14).
Although the relay’s contact point is rated 120VAC, you should stick to low-voltage (<40V) devices, because the PCB traces and enclosure are not rated for power line voltage. Garage door openers typically run on low voltage so they are fine. The only reason for 120VAC relay is that it's literally impossible to find relays where the contact rating is less than 120VAC.