Shadows

In toon and realistic lighting modes, the shader will receive shadows from cast lights and self-shadowing.

Shadow Strength

Controls how strong the shadows will be. This includes shadows determined by how much of a light a pixel sees and shadows that have been cast onto the avatar.

Disabling this will not disable shading, technically. The avatar will still receive indirect lighting (light probes) and, if realistic lighting is enabled, specular reflections.

Receive Shadows

When enabled, the material will receive cast shadows from itself and other objects in the world.

This only effects shadows from directional and additive lights.