The small port on the lower left side of the FM-206 face is an equipotential port. This gives you easy access to a ground point to allow you to integrate the FM-206 into your ESD work area to ensure there is no potential between the grounded tip and the PCA you are working on. The proper use of this jack would be to use a grounding wire with a 4mm banana jack running from the port to an ESD worksurface or workstation bonding point. If you do not have an ESD bonding point available, you may use the port at the front of the FM-206 to attach your ESD wrist strap.
If attaching a wrist strap, be sure you have at least a 1M ohm resistor as part of the wrist strap.