The abstract concept of the person who uses an item to accomplish some operationally relevant task. Being abstract, this term is uniformly applied to mean both operational users of the product, and some duly recognized organization representing them.
The parameterization of the task may, or may not, be directly related to that of the design’s features.
As a concept, End User can cause quite a bit of confuscation: an item may be used by an End User to develop or produce some other item for some other End Use. The System Engineers have to avoid noodle baking in such situations.