I just noticed that the terminology I used was very unfortunate. I was using "emission" from an older time period where emissive pixels were added to the scene on top (as an additive or "screen" overlay in Photoshop), but not multiplied with the scene in any way like realtime GI or anything like that (as a light would be multiplied with the albedo/diffuse color of the scene).
Harald ：The emissive parts are lighting the surrounding environment already, so it should not require a second light to lighten the environment.
Here I was sloppy with the wording: Instead of "lighting the surrounding environment" I should have written "brightening the pixels below it". You are right that this will not influence the environment like a light would, it just makes the area additively brighter, but will not replace a light and cannot bring out details again in an unlit background.
The intention is to have the attachments with additive blend mode separately controllable (making them brighter or darker) from a separately placable light that affects the scene. So you can e.g. make the additive texture very bright, but at the same time have the light intensity low to not mess up the contrast.
While talking about it, I think we should add a material property HDR color Additive Color where you can tweak both the color and the intensity of all your additive attachments at once.
