Categories are how readers find an article after they know the topic. The service desk tree is only for this library.
Where the tree is used
Open the tree at /app/knowledgebase/tree. Categories you add there are the ones offered when you create a service desk article. They do not change the Trust Centre category tree, and they do not change ISMSVision product help.
Service Desk Help uses the same tree so a reader browses the structure you published.