thanks for the reply. even the expert at the camera store didn't know this.
The camera body has a white focus assist LED to help focus in dark places. It is not very bright, and so has a short useful range (maybe ten feet), and it also can disturb the subjects flashing in their face (the flash flashes too of course).
Many hot shoe flashes have a red infrared focus assist LED that kicks in instead (it turns white camera LED off). Different flashes have different patterns, but often a grid of dots. It looks dim because our eyes can't see infrared (we see a little red). But it is actually much stronger, and is effective much further. I don't know how far, but 40 feet is easy with it.
It is slightly tricky to get either one of them enabled. The camera should be in AF-S mode, with a Single Point focus selected. Otherwise, neither light will likely ever be on. Same procedure as the camera manual documents for the white focus assist LED.
FWIW, you can see three focus assist patterns in the three speedlight reviews in link at the sig below.