Timesheet amendments can be requested when the timesheet has been approved incorrectly, and can be requested by the client or the agency.
You can request an amendment to an approved timesheet by finding it in the Approved tab and selecting the option to amend.
Once requested, the amendment will appear in the Amendments tab on the timesheets page, where you can track if it's been approved or rejected by the client. If the other client requests an amendment, it'll show as "pending" in this list, and you can approve or reject it there.