This page describes the behavior of all Mixed Lights in a Scene, when you set that Scene’s Lighting Mode property to Subtractive.
In Subtractive Lighting Mode, all Mixed Lights in your Scene provide baked direct and indirect lighting. Unity bakes shadows cast by static GameObjects into the lightmaps. In addition to the baked shadows, one Directional Light, known as the Main Directional Light, provides real-time shadows for dynamic GameObjects.
Because shadows are baked into the lightmaps, Unity doesn’t have the information it needs to accurately combine baked and real-time shadows at runtime. Instead, Unity provides a Realtime Shadow Color for reducing the contribution from the lightmap to create the illusion of a correct blend between baked and real-time shadows. You can also tweak the color to achieve a certain artistic style.
Subtractive Lighting Mode is useful on low-end hardware where performance is a concern, and where you need only one real-time shadow casting light. It doesn’t provide particularly realistic lighting effects, and is more suitable for stylized aesthetics, such as cel shading.
When you set a Scene’s Lighting Mode to Subtractive, Mixed Lights behave as follows.
When you set a Scene’s Lighting Mode to Subtractive, Unity displays the Realtime Shadow Color property in the Lighting window. Unity uses this color when it combines real-time shadows with baked shadows. Change this value to approximate the color of the indirect lighting in your Scene, to better match the real-time shadows with the baked shadows.
Unity automatically chooses the Directional Light in your Scene with the highest intensity value to be the Main Directional Light.