https://www.groundzeromedia.org/show/7-2-24-ufo-canon-the-doctrine-of-eternal-recurrence-w-donald-r-schmidtt
https://www.roswellufomuseum.com/don-schmitt
Don Schmitt is quite an authority on this.
https://cufos.org/about-us/cufos-board-consultants-and-friends/donald-r-schmitt
https://cufos.org/hynek-biography/j-allen-hynek-biography
"His job was to uncover secrets from the unknown. But his curiosity opened doors that the government preferred to keep closed." https://www.popularmechanics.com/military/a60167697/j-allen-hynek-project-blue-book-ufo-investigation-revelations
https://www.vice.com/en/article/zmbj33/the-professional-ufo-skeptic-who-believed-in-aliens
https://www.history.com/topics/paranormal/roswell
The Roswell UFO crash is purported to be a physical spacecraft with occupants. However, any ET crash would officially have to be denied.
https://www.history.com/news/roswell-ufo-aliens-what-happened
https://en.wikipedia.org/wiki/Interdimensional_UFO_hypothesis#Hynek_and_Vall'ee
UAP is a much more vague and general term, which could be applied to almost anything.
https://en.wikipedia.org/wiki/Bioship
ET would imply a physical being or creature from another world, but likely from the same universe.
https://now.northropgrumman.com/do-doors-to-interdimensional-travel-exist
Various science-fiction stories have included the Inter-dimensional_Entity (IdE). Such beings are from another dimensional plane of existence.
https://lovecraft.fandom.com/wiki/Russell's_Guide_to_Interdimensional_Entities , https://www.merzo.net , https://lovecraft.fandom.com/wiki/Interdimensional_Entities,_Artefacts_and_Incantations
"Astrochicken, Dyson explained, would be a one-kilogram spacecraft unlike any before it. It would be a creation of the intersection of biology, artificial intelligence and modern microelectronics—a blend of organic and electronic components." https://en.wikipedia.org/wiki/Astrochicken#Description
https://en.wikipedia.org/wiki/Space_animal_hypothesis
https://cufos.org/types-of-ufos/triangular-ufos
