Complex sentence: Ultra long-haul refer to the flight length of a flight being "ultra-long", which while there are no universally accepted definition for what is considered as ultra-long, but they generally mean flights that are 12-hours or longer.
Split into simpler sentences: Ultra long-haul refer to the flight length of a flight being "ultra long". While there is no universally accepted definition for what is considered ultra long, the term generally refers to flights that are 12 hours or longer.