Catalog 
A catalog simplifies the rule maintenance process for the business user by hiding the repository and displaying only the relevant objects that are necessary for the maintenance of business rules in a given business scenario. It serves as a filter for the entirety of all objects that exist in the repository.
Recommendation
To take full advantage of the complexity reduction that you can accomplish by using catalogs, we recommend that for business experts working with BRFplus, you disable all other views in the BRFplus workbench so that only the catalog view remains. To do so, choose 
Workbench
Personalize Views 
and disable all views except Catalog.
A catalog comprises structure nodes, object nodes, and link nodes. These nodes are used to define the structure of the catalog.
Structure nodes allow you to create a folder structure in a catalog. Object nodes have references to BRFplus objects like expression or function. Link nodes have references to nodes from a different catalog, thus allowing you to embed already existing catalogs into a given catalog.