A label lets you further identify the catalog. You label a catalog when you create it. Applications and administrators can use the label to identify which catalog to use. Several applications can share the same catalog.
You have to use both the primary and the secondary label. How you label a catalog is up to you. For example, for a primary label, you can use your company name or your department (if it's a department-specific catalog). The secondary label can be anything that further distinguishes this catalog from another one.
WARNING: Some labels are created by the application with which they are associated. You should not modify these labels, as the application uses them to find out which catalog to use.