How to define new categories or sub-categories in Change Management section of servicedesk plus?

How to define new categories or sub-categories in Change Management section of servicedesk plus?

Under ServiceDesk --> Changes --> Category
 
How can i define new categories or sub categories? e.g. Under category pull down menu, you get "Telephones" or "General", how can you add sub categories under each one of them. Also, how can you add a new item to category list itself?
 
 

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