If you need to restrict a user to one category, you can use the Live Filtering feature in Cumulus user permissions.
STEP 1: In this example, I create a new category called "Texans", and then do a custom search for "Categories" -> "Contains" -> "Texans". This will restrict results to show only assets in the new category. Use the pulldown menu on the upper left to save your query.
STEP 2: When saving, click the checkbox for "Share Query", this will make it available for other users.
STEP 3: Go to Server Console -> User Manager -> (Select User) -> Properties -> Catalog Permissions. Enable general access to the catalog, then scroll down to Live Filtering. Click checkboxes for "Use Category Live Filtering" and "Use Record Live Filtering"
STEP 4: Click the "Use Query" button, and select the search query you created earlier. Set for both "Use Category Live Filtering" and "Use Record Live Filtering" then click "OK" twice to save your new user settings. Return to Cumulus, login as the restricted user, and you will see only the restricted category.