The term "endurance" is borrowed from motorbike racing and it comes somewhere between downhill and trekking.
It is carried out on technical downhill courses with a certain number of ascents. In general endurance is associated with the mountains.
Endurance MTBs need to be both comfortable and high-performance machines.
Endurance MTBing takes place on downhill circuits. It is all about pleasure, friendliness, discovery, and autonomy.
Endurance is based on technical expertise, managing effort over time, and versatility.
It is these values which set endurance apart both from downhill and from cross-country mountain biking. Whilst attack is of course one factor on an endurance course, it is the technical dimension which is decisive.
Equally, whilst there is a physical aspect this is not an end in itself. In concrete terms that means it is wholly feasible to have sections in which the participants need to peddle, and climb even. The difficulty of the course needs to include physical section as a selection factor (since endurance requires versatility), but without these sections being all-important.
In order for endurance to establish itself as a discipline in its own right, distinct from other closely related forms of MTBing, it needs to find the right balance halfway between downhill and cross-country - without veering too far towards either.
At the same time the courses are designed to be enjoyable to follow and discover. That is why they tend to place the emphasis on fun and follow rough tracks for the great majority of their length.