In addition to being a convenient way to group together Tasks of similar functionality, area of business, or purpose, Categories are also useful for controlling which users has access to which Tasks.
You can edit a Category's security settings by launching the Category Options Dialog and clicking "Edit Access Restrictions". By default a Category is public, meaning that any Presence user can open the Category, browse and edit Tasks within that Category, however it can be changed to Private (meaning that only certain users may access it) by clicking the radio button that says "Private Access".
The dialog will then be expanded to show an inclusion list of users:
Simply drag and drop users from the "Excluded" pane to the "Included" pane to allow those users access to this Category and the Tasks contained therein - or vice versa.
Note that access restrictions are propagated to sub-Categories.
Users whose role is "Restricted" have the right to open and browse the contents of a Category but not edit Tasks within it.