Each assignment defaults to the employee's standard charge out rate. You can change that rate under Rates tab as well as assign an expiry date to it.
You can also use this tab to assign a rate type (e.g. contract rate) to a particular assignment so that time spent is charged to the contract rate rather than the standard rate when the employee completes their Timesheet.
You can use the - UNSPEC- employee to assign the special rate to any employee working on the assignment.