The calculation type will need to be set to 'Window Calculation'.
You will need to make sure you added 'Category' to the dependency pane on the calculation window and you need to modify the Incident window and set the 'Is Calculate on Change' property on the category attribute to True. This tells the calculation to run when the value in the Category dropdown is selected.
The attribute with the calculation on it must be on the window (it can be hidden by setting the show on window property of the attribute to false) and the calculation must not be on the source or target attributes. Dynamic window calculations should be created on a separate String -1 field used to store that calculation only.