Catalogs are held in packages and much like Character Groups, they are composed of a title and description. You create catalogs on the package window and then open them into a new window to add items to them. Catalogs can contain skills, melee and range weapons, general items, custom rule references, media, dice roll configurations, tables and print templates.
Organization of your catalogs can take any logic you need Most CoC catalogs are broken into eras but that certainly doesn't have to be the way you do it in the CoCGMS. A Package can contain any number of catalogs.
Catalogs can be linked to other packages, any character group, any scenario or directly to characters in the system. Characters inherit catalog links from their chain of ownership (Package --> Character Group --> Character, with Scenario linked catalogs being thrown in for additional access). Only content from catalogs linked to some point in the character chain will be available to add to character inventories.