FBX Export: What does Forward / Up assignment do? I want to bring my object from Blender 2.8 to Unity.
I'm experiencing that the object lays on its belly in Unity.

To find what is actually happening, I'm trying to find out what the following settings actually do:



However, I didn't find this info.

Here is what my Blender scene looks like:



To me, the object looks adequately oriented, so I don't understand what Blender actually wants to know from me when it allows me to assign Forward and Up axis at fbx export.

I would like to actually understand it and not just try it until it works accidentally.

Thank you!
Each software handle axis names/orientation differently. That doesn't change anything while your working into a soft, but can be problematic when you export models and animations.

Z Up / Y Up

In Blender, the vertical axis is called Z (so X and Y define the floor plane). In many other software like Unity, the vertical axis is called Y (so X and Z define the floor plane).

Y Forward / -Y Forward

Once you know which axis means vertical, you need to determine how the two other axis are oriented. 



In Blender if you turn the view so the Z axis points up, and the X Axis points right, then the Y axis points backward ; in other terms, the -Y axis points forward.

Sometime this kind of orientation is called "left-handed coordinates" :


Of course it opposes to right-handed coordinates where the "Y-finger" would point forward.

Blender / Unity

Blender is Z Up / -Y Forward

Unity is Y Up / - Z Forward

Other cases

It's quite conventional that the vertical axis points up (not down), and the X axis points right (not left). However, that's not universal, so the fbx exporter allow every valid Up/Forward combination. (an invalid combination would be something paradoxal like : Z Up, Z Forward).