Usually one article will be added to only one category in your help center. However, there might be cases where you would like the same piece of content to appear under multiple sections of your help center so it's easier to discover.
In this case, you can attach an article to multiple categories.
To do so, open that article in the article editor. From the sidebar with options on the right, under the Category field, toggle the Multiple Categories option on (it should become green).
Immediately, from the select menu, you would be able to choose any of the available categories and attach the article to the selected one by pressing on the Attach button.