Overview
We rely on Assortment Groups when you have a specific organization where you have a team that plans eCommerce completely independently from the team that plans stores. You don't want your Assortment mixed together with other channels, you want to be able to physically separate your assortments based on your Assortment Groups and have no other planners have an affect on your channel specific assortment.
Another example would be you could have a combination of Web and Stores vs Wholesale with its own inventory.
In the simplest case, if you plan all channels at once, you'd use the default "All Channels" option where all your channels are planned together within an assortment.
Assortment Group Configurations
In order to start using Assortment Groups for your assortments,
Step 1
Either CMD+K and search for 'Assortment Groups' or
Go to Settings > Configuration > Module Settings > Assortment Plan > Assortment Groups section
Click on ADD button on the top right
Give your Assortment Group a name and Save
You can edit and delete the Assortment Groups that are created on this page as well.
Step 2
Either CMD+K and search for 'Clusters' or
Go to Settings > Configuration > Intelligence > Clusters
Double click on Assortment Group cell of the relevant Cluster that you'd like to associate your Assortment Group with and select that option
Click on Save for your changes
Creating Assortments with Assortment Groups
Please follow the instructions here for creating assortments.