In the context of 3D rendering and material properties, the term "emissivity" refers to the luminous/light-emitting property of a material. Emissivity can be applied to any ordinary material in the design to make it emit light in the renders. At the same time, it is important not to assign too high a value, in order to avoid glare. Here's a breakdown of recommended emissivity values for different types of lights:
Ceiling Cove Lights: For recessed cove lights, an emissivity ranging from 1-5 can be assigned to the recessed ceiling slab that is intended to emit light.
Back Lights: For concealed backlights such as those applied behind wall mirrors or wall paneling, an emissivity ranging from 10-30 is recommended, depending on the degree of glow you want to be emitted from behind the panels/mirror.
Strip Lights: Thinner elements such as strip lights are recommended to have an emissivity value of around 10.
These values are merely recommended ranges for different types of lights you can apply into your design in common design cases, and are by no means applicable in ALL cases. The emissivity value assigned to a particular light is dependent on the intended luminous effect, combined with the brightness/darkness of the surroundings, and the colors used in the design. For example, for deeply recessed backlights in a dim room, the emissivity required may even go up to 70 or more.