The ENETS 2023 guideline for dNETs are combined with the guidelines for Gastric NET (gNET) due to their close relationship in anatomical terms.  But there are functional linkages to the pancreas, therefore also links to that as above.  

The duodenum is technically part of the small intestine, and you can find that in most anatomy-based descriptions. One of the research references I used was titled “Somewhere between the pancreas and the small intestine” and in the case of Duodenal NENs (dNENs), I can understand why they adopted that unconventional anatomy description.  I suspect they contextually meant that they are not like either of these organs, but they share some similarities.  This seems at odds with the ENETs decision to link guidelines for stomach and duodenal in one. 

They are not normally treated like small intestine NETs (SI NETs) or pancreatic NETs (pNETs) but they share some relationships, particularly with the latter.  They are also closely aligned to the stomach (Gastric NENs) in terms of symptoms and syndromes that they sometimes present.  Although they are a type of NEN (i.e. dNENs), they can be grouped together with the stomach for certain scenarios, e.g. Gastroduodenal NENs. This might be more aligned to ENETS guideline structure. 

dNENs are classified into 5 clinical sub-types: non-functioning, functioning consisting of gastrinomas, and more rarely in somatostatinomas and serotonin-secreting.  Also rarely as duodenal paragangliomas and poorly differentiated neuroendocrine carcinomas (NECs).