A set in Maya is a collection of objects or components (for example, geometry objects or lights).

Grouping objects into sets allows you to make certain changes to multiple objects at once.

MtoA can use Maya sets to apply Arnold rendering overrides to the objects inside. This allows you to override any attributes of the objects that are members of the set, which means you can affect a large amount of objects without changing the properties individually.

* Maya Arnold light Group using aistanderdSurface Arnold 2.0

We can create Light group  using Alshader
You want more control over the lights. Wanna create separate light passes then Light Group Aov is the only option

 (In latest Version of Arnold 2.0 for Maya Release Date April 12, 2017 U can create Light group without using any 3rd Party shader such as Alshaders)

* Maya Arnold light Group using alshaders

* Maya Arnold Override Sets

In latest Version of Arnold 2.0 for Maya Release Date
April 12, 2017 U can create Light group without using any 3rd Party shader such as Alshaders
You want more control over the lights. Wanna create separate light passes then Light Group Aov is the only option

Maya - Arnold -Katana tips