As far as I can remember using the negative values actively back in 2014/2015, we had as low as -5 (minimum available), however I don't recall ever seeing -9 (at least not in the ROW environment).
https://wiki.waze.com/wiki/images/3/31/ ... vation.png

I took a look at the area you were editing and it looks quite complex, if you just use Ground, -1, -2, -3 and -4 for cases where 5 segments (1 Ground + 4 Underground) have intersecting WME geometries isn't that doable?
More underground levels would mostly be needed for cases where you have 5 or more intersecting underground segment geometries.

For example, this segment has only 1 other which crosses its own geometry, the other is set to ground, so it could simply be set to -1. Right now it's set to -3 but there are no intersecting segments at -1 or -2 above it, just the ground segment so to me it looks un-necessary to use -3.

Ground level segment	Ground level segment
No segment	-1 segment
No segment	No segment
-3 segment	No segment

You may be just following some US Wiki rules or guidelines I'm not aware about.. So only take what I write as a personal opinion as I may be missing something and am not an expert on this topic.
I always took those values as a way to indicate to the WME which segment goes above/under others when you stack them, rather than a way to say how many levels below ground anything is, if that was the goal we would probably have gotten a tool to enter a value to represent altitude or depth in feet or meters instead.