To make this easy to figure:
You see what is reflected in that chrome kettle?
So the texture it needs to be put in the Ambient light URL is exactly the image reflected in that kettle.
(Highlights from sun and spotlights excluded)
So you need to provide spherical texture that will fake as close as possible the environment where your objects are. But keep in mind that it has to be enough generic to be used for all the objects that are present.
If you don’t provide that texture, it will fallback on the skybox texture (which can do the job for outdoor scenes, but become less accurate when your objects are inside a building.
There is 2 spherical texture… the skybox and the ambient light, so it allowed you to have a sky that you can see outside but having a more accurate Ambient.
A good tool to generate ambient light texture is definitely the new 360 spherical camera release a few weeks ago.
You will understand that this faked solution is used because the computing of this wouldn’t be performant enough for real-time rendering.
It could have been partially pre-compiled (like Sansar and Unity probably do) but in High Fidelity, we want the world to be modifiable live. So this is the sacrifice made to preserve that ability.
In any case… you can hope to see your face in the kettle, not here, not even on Sansar… this would be definitely too much processing.
Ambient light Intensity…
This is simply the factor apply on the quantity of light returned. This will affects light degree that you will see on the part of the object that is in the shadow. (As the dark side of the moon)
When you are inside, and check the side that is not exposed to the light, it’s not fully dark, it’s still exposed to the light return by the wall and it allows to see those surface.
At the opposite, you are on the moon, the surfaces that are in the shadow appears very dark, because there is not too much light reflected by the sky that is dark.