Warning message when posting to Revenue Elements

Hi Gurus,

When i am posting any account entry using account which revenue element, system is showing warning message which says enter true account assignment object.

We are not using COPA. and as per my understanding we will not enter cost center also in revenue elements, right??We will just enter profit center, as we have document splitting on profit center.

Please let me know in which case we use revenue cost elements if we are not having COPA. And if system is giving this warning if we are entering it with profit center, is it fine?

Thank You