Ummmm.. 'endurance' is just that - to tanks empty. In what is probably long oudated parlance, it's when ALERFA becomes DISTRESSFA - i,e., the aeroplane can no longer be in the sky (unless very briefly, with a reassuring swishing sound outside the window familiar to any glider pilots amongst us).
If it's not that, how in the hell do you give an 'endurance' figure to ATC without giving them chapter and verse about your diversion options?
In 40+ years in the business, this is the first time I've ever heard anyone say that endurance is anything but to tanks dry. That's what gets written on the ICAO flight plan.