Distribution key in KP06

While planing activity independent primary and secondary costs, distribution key is required probably in order to distribute the annual costs to the posting periods. Is this logic correct?

And if this is so, then in variable activity dependent costs, distribution key should not be required because variable activity dependednt cost will be directly proporstinal to the output activity of the cost centre. But in KP06 (as per the CO manualTFIN20-2 MA I), distribution key is also given in case of such costs? Why? I understand that those variable activity dependednt costs should not be divided in posting periods equally but on the basis of activity output in each posting period. Then why distribution key is required?

Please help clarify my concepts

Asif, Karachi Pakistan