Original W126 Euro Bosch Headlights

[INSIDIOUS]

Original W126 Euro Bosch Headlights Gen I 4 door. These have the H4 55/60W set. I notice there is another tiny bulb down near the bottom center. Not the fog bulb off to the side. I tested the bulb and it works, but I just can't figure out when it is supposed to be on, it seems to never be on no matter what I am doing with the lights. Is this some sort of daytime running light, or what? Thanks!