In such situations, Chief of Staff is basically an Executive Assistant, but in big companies, an EA is not a mere secretary who manages the calendar and orders

In such situations, Chief of Staff is basically an Executive Assistant, but in big companies, an EA is not a mere secretary who manages the calendar and orders food and books flights. An EA basically becomes the right-hand man/woman of the exec. Also, Chief of Staff is not merely a Valley term. It's common in DC.