Antonio,
The revenue earned by the employee is determined by the Time and Expense Assignment to a particular Sales Order, and the eventual Customer Invoice based on that Sales Order. I'm not sure what you're referring to when you mention the "Sales Unit Text box".
When you say "cost of the employee", there's two parts. The actual value of the time is dependent on the labor resource determined by an employee's job and cost center.
The cost center that the time debits is based on the Responsible Unit of the Project, yes. The employee's cost center is credited for the time. So as to the question "which cost center does the employee's time hit", the answer is both, but for different purposes as described in my original response.