A catalog group is a set of catalogs that you can jointly assign to a group of customers.
A catalog group is one or more catalogs that you make available to different customers or organizations. You can include base or custom catalogs in a catalog group, and you can control which customers have access to which catalog group. For example, if you have a frequent buyers program and you give the members of this program discounts or incentives that you do not give to the general public, you can create a catalog group with specific catalogs in it, and then let only the members of the program access those catalogs.
If you have products in separate catalogs and you want to give access to those catalogs to a few select customers, you can make that catalog part of a special catalog group to which only those select customers would have access.