Hey Jeff,
you are right, there is something connected to SC2. This is the TTL level output version of the optocoupler output version for “CW”, SC5. I did not know that, and I do not know why they use the pin. Also I cannot see in your image to where this wire goes and what the overall circuit is. I don’t own and will never own a MASSO G3 controller. You should ask support@onefinitycnc.com for what this is for. Possibly it is for Emergency stop wiring. But connecting the other pins as described while at same time this wire is at SC2 will probably not impact functionality of your wiring.
Note that in the official Onefinity support document “Support: VFD/SPINDLE wiring schematic for Elite Masso machines”, this connection is not documented.