Custom or skybox ambient lighting data.
Skybox ambient lighting mode uses this Spherical Harmonics (SH) probe to calculate ambient. You can also assign a completely custom SH probe this way.
The GI system will bake the ambient probe, but it actually won't be used on geometry that uses light probes or lightmaps, as the environment lighting is already in the light probes and the lightmaps. It is used as the last fallback if light probes or lightmaps are not present or enabled for an object.
Adjusting the ambient probe will not affect the input to realtime and baked Global Illumination. If you want to adjust ambient in a way that affects GI, adjust ambient through RenderSettings.ambientMode, for instance by using AmbientMode.Trilight. The GI system will output the resulting ambient values into the ambient probe, which means that a custom ambient probe can be overwritten by the GI system.
See Also: ambientMode, SphericalHarmonicsL2, The Lighting window.