If looking for simple “on/off” control with a relay, look for a simple digital on/off pin. In my experience, this is often described as the “spindle enable” signal.
I built out a full closed-loop variable RPM solution using the AC dimmer board sold in the V1 shop. It works, but it is not a single board, turn-key solution. This experimental project is covered in the extensive PID -Hardware Solution needed for a software fix discussion thread. For CNC control it requires an additional microcontroller (I’m using an Arduino Nano) that runs code (I had to tweak to work correctly with my non-V1 control board) which monitors an RPM sensor added to the spindle which then mediates the PWM signal coming out of the CNC controller and turns it into the analog voltage input the AC dimmer board needs, and to tweak this analog voltage to maintain the commanded RPM under changing cutting loads. I added a switch and a potentiometer to give me manual RPM control as well. I like the solution, but I like to tinker and it’s way more intricate than simply identifying an on/off pin for a relay.