Scala Class Definition A <: B : C [duplicate]

I'm new to Scala. I tried to search this syntax but I can't find any example because I don't know what this is called.

The class definition looks like this. How do I understand this? I understand the conformance part that type A conforms to type B, but what is C there for?

class Hello[ A <: B : C ]
  extends D.Apply[ InputType, A ] {
    // ...
}