Emissive layer block is pretty simple, as the diffuse one:
It lets you specify the light color and its intensity. Koru use these parameters when it computes the scene glow effect. Any reflected highlight or emissive layer block adds to the glow effect.
Below is a scene of three balls. The left one has simple diffuse gray material, the middle one has emissive block enabled and the right one has emissive block with checkers texture to control the emissive areas.
You see the left ball doesn’t glow at all, the middle one has even glow, while the right one has more glow on its brigh areas.