It is technically the standby reticle as mentioned above but in house it is known more as the seat height marker due to that being the most common use for it.
As soon as you apply battery power the pilot will adjust their seat height lining this up.
Short guys and girls need not lower the seat.
Taller people lowed the seat quite a bit.
You must always check the leg restraint line anchor points are flush with the floor so they don't pierce the rocket motor of the ejection seat.
The HUD is not a display that fails that often so not many people know what this thing is for.
The fact that you can just bring up the HUD page on any of the other displays when it fails and beyond visual range weapons being the default way with regards to tactics means it is seldom used.
Can't give any references as the publications are restricted, but I was a maintainer on this aircraft for over 10 years.