The Category status is used to mark bill items to ensure only one item can be added per bill this is sometimes a requirement for third parties or rebate.
To mark the bill items as Category, go to Administration - > Global Catalogue.
Click the lock (found at the top left next to the save button).
Use the category checkboxes to mark any needed items.
Once changes have been made the Save button at the top of the page will become clickable and should be used to save any changes.
Any bills that have already been raised that should have category items will also need to be edited.
Then with the item you want to edit selected, check the Category box and click save.