The Package Layout
Clicking the "New Package" button from the Main Menu will create a new empty package for you. You'll be prompted for the name during creation but it can be changed at any time. Inside of a package you can create related scenarios, catalogs, character groups (and characters within those groups), table concatenations from any tables in the system and more.
At the package level you can link catalogs from any package in the system to give your characters inventory to select from. This means you can link catalogs from the base packages like the Chaosium and CoCGMS Add-on packages or any other package.
Tags are descriptive terms you define at the package level which are then available to other areas in a package. Many of the lists and tables in the system can be filtered on free-form terms and tags can help you find related pieces of information.