Lights - settings - Arnold for Maya
See Render Settings in the Arnold User Guide
These settings provide some general control over how lights are evaluated in Arnold.
Global Light Sampling
When enabled, the light sample settings on individual lights are ignored and the overall contribution of all lights together is computed using only the specified number of samples.
It does not apply to environment, directional lights, volumes, and GPU rendering.
Low Light Threshold
A low_light_threshold can be set, which can speed up rendering by allowing Arnold to skip the evaluation of light samples when the amount of light is below a certain value.
Normally, all lights in the scene can illuminate all objects in the scene. However, Maya allows you to override this by 'linking' lights with specific objects so that a light will only illuminate an object that it is linked to. By default, MtoA is set to follow whatever light linking is defined in the Maya scene, but you can override this. MtoA also lets you separately control whether shadows behave the same way.
Light Linking
Turn off light linking ('None') or use Maya's light links ('Maya Light Links').
Ensure that Light Linking is set to none when instancing lights. Otherwise, the instanced light will not render.
Shadow Linking
Shadow linking can be set to be the same as the setting for light linking ('Follows Light Linking') or you can specify explicitly that shadow linking should be turned off ('None') or use Maya's shadow linking ('Maya Shadow Links').