Dial Indicator Rod
On the right side of the head of my manual knee mill there is a rod that sets in clamp in the casting. A simple thumb screw holds it at whatever height you set it. Its pretty short. In fact its kind of in the way. If it was any longer I would have already removed it and tossed it in a tool box.
I'm not really sure what its good for. Not really suitable for a tapping head arm if you use the quill. I can't imagine tapping with the knee. Maybe it might be okay with a coaxial indicator, but that seems a bit of overkill to build a stop rod into the machine. I'm also not sure it would reach down far enough to engage the arm on my coaxial indicator.
Generally I think its an "anything you can use it for" feature at best.