ECOs are used to make changes to the device if the SOP’s , suppliers, processes have been changed. ECO’s are set into place once the design controls are completed. ECO’s generally cost a large amount of money to actually process, initially all departments associated with the design controls must agree on the reason behind the request. Personally i have no yet encountered a situation where a ECO had to be requested. In the company i work for, they search for other avenues to solve the problem before having to resort to a ECO. But as many have pointed out, ECOs are very costly and this is the reason why it is heavily avoided.