Seed of Unknown Form
The Seeds of Unknown Form (正体不明のタネ shoutaifumei no tane) are small snake-like flying objects with no predetermined form. Because of this, when a person sees it, what they see depends on their own experience and knowledge. According to Nue Houjuu, they can appear as pieces of wood, birds, or even as danmaku.
Nue Houjuu places the seeds of unknown form into the fragments of Tobikura, one of the elements needed to unseal Byakuren Hijiri in the events of Undefined Fantastic Object. Because of this, the fragments scattered into the air. (It was initially believed that it was because of the geyser in Subterranean Animism, but Nue reveals the facts in her dialogue.) They take the form of unidentified flying objects (UFOs) that bounce around the edges of the screen because that's how the playable character saw them. They ended up collecting them, much to the delight of Shou Toramaru's group of youkai.
The game system of Undefined Fantastic Object makes use of these "UFOs" in a sort of color-matching game. They are the game elements responsible for dropping extra life shards and bomb shards, as well as points. See Undefined Fantastic Object/Gameplay for details.
- Sometimes also translated as Seeds of Non-identification.