AO distance tells this is related to AO, doesn’t it?
Also each tooltip is quite self-explanatory and refers to AO.
The reason why it is not decoupled I think is that it would imply a double AO calculation in case one wants to use regular AO for rendering, or have an AO in output passes.
Yet it would be nice to be able to set an override background for it independent from the world settings.
This way you could have an HDR or whatever you like in the background and still manage strength and tint of the fast GI approximation. Tinted light is often an issue with this cool feature