fred wrote:
Tuned length termination stub sounds good to me.

All this makes sense, except... if you look at both sides, the other end of
the line doesn't connect to anything. It could possibly be a stub for a
buried trace (into a via in the hole) - if a signal was sandwiched between
two power planes and then emerge to the surface here. But otherwise I'm
having difficulty thinking what you'd put through a cable just to use a bit
of PCB as a stub/antenna. Particularly using a pin header as the connector.

But... maybe it's a manufacturing test. Connect your network analyser to
the SIM1 or SIM2 pins, check that the board fab is within spec. For PCIe
the board parameters matter quite a bit, so if the layers were the wrong
thickness this would be something this test would detect.

That's quite a neat idea. I might try it next time a do a fast board.

Theo