Selection lists for category fields are determined by a system's Category lists.  Category lists include non editable options that are native to the system as well as editable custom content.  The following example sets the Category field to use the options store in the Content Types Category list.
<Category Id="MailState" Name="State" CatType="State" Required="true" Multiple="false"/>

The example above creates a field 'MailState' which will draw options from the Category List: State.

Configuration of Category lists can be performed by an Administrator.  On the Admin menu click Categories.  Click on any Category to access and manage the list of options for that Category.