In general, Cost Centers are not meant for revenue posting. They are meant for expenses. However, you can allow them by your cost center category. But, this is not industry best practice.
Cost center, WBS element and Internal Orders are real CO objects, where the real postings will flow.
Profit Center is always statistical only (statistical means just for information postings) - These postings are like shadow postings.
When you give two real CO object for posting like Cost Center and Internal Order (Real), then real postings will go to Internal Order (Real) and statistical postings will flow to Cost Center and the profit center assigned to Internal Order. Cost center will modify itself to a real or statistical based on the other CO object given.
Hope this is clear.