For those who don't know, the helmet display can be custom set to show specific symbology. If the HMD has been turned on and BIT checked, then the HMD button is available on the DDI SUPT page. There you can set the REJ level similar to the HUD. Additionally, you can click the REJECT SETUP button to choose what is displayed on each level.
I'm not sure who else might be looking for this, but I noticed today that the settings labeled as WINDOW 1-10 now have some function.
I've been offline for the last few updates, so I'm not sure when this was implemented. The Chuck's Guide dated 21/08/2021 (page 352) shows these as No Function in DCS.
What I've found out so far through trial and error:
Window 2 - RDR/FLIR - Displayed when sensors are slaved to radar/FLIR
Window 3 - MAV/CCIP/etc - Displayed when ATG weapons are selected
Window 4 - GUN - Displayed when gun is selected in ATG mode
Window 7 - ATC/NWS - Displayed when throttle lock or NWS is activated
Window 8 - target/waypoint distance - Displayed when HSI is tracking steerpoint
I haven't found what the other five do as of yet. If you find out, then post it here. Maybe someone has this info already, but I haven't been able to find it.