Conjuration/Emission/Transmutation

These three Hatsu types tend to cause a lot of confusion to some people. To put it in short, a conjured thing is solid and material, anyone, whether or not they can use Nen can see it (unless In is used to conceal it); a transmuted thing is pure aura that copies the properties of other things, whilst emission abilities shoot pure aura able to be kept distant from the user's body.

Kastro's Doppelganger [Conjuration] - The Doppelganger was solid, ordinary people could see it, and it could be separated from the real Kastro as long as it stayed relatively close.
Hisoka's Bungee Gum [Transmutation] - Normal people are unable to see it and it always is physically connected to Hisoka. Furthermore, it is not solid, but rather pure aura energy copying the properties of rubber and gum.
Razor's 14 Devils [Emission] - They were not solid and ordinary people couldn't see them. Razor's 14 Devils were pure aura, and they did not need to physically stay in contact with him.