Andy also explains it here:
forum.linuxcnc.org/27-driver-boards/3537...to-mesa-7i76e#119013
The VFD provides an isolated GND (ACM = analog common) and 10V output (preferred) to the Mesa card (Spindle- / Spindle+). Spindle out is a potentiometer, like you said.
In this case the input voltage is 12V and that needs to be scaled down to 0-10V. This happens in your INI.
Take GND also from the analog section.
Enable and direction are isolated outputs allow sinking, sourcing combinations of both. Most chinese vfd are NPN inputs - you need to provide GND (DCM = digital common) to their input to close the circuit.
You take the GND from the vfd and connect it to SPINDLE ENA- and from SPINDLE ENA+ back to your VFD (input). That way you are providing GND to the VFD input, when the relay is on. Same for the direction.
Most VFD's have ACM (analog common) and DCM (digital common) but I couldn't see that in the A2 manual.
SPINDLE ENA- <-> VFD GND
SPINDLE ENA+ <-> VFD FWD/RUN
Say your inputs are PNP (24v)...this would require a 24V signal. You would take 24V from the VFD to Spindle ENA+ and back to the VFD on SPINDLE ENA- to your input.