What is the difference between braced and parenthesized item

Stream: new members

[Ben]

I have seen two different syntaxes between declaring parameters def {A: Type} ... and def (B: Type) ... . My current understand is that in the former the A parameter is inferred (isn't specified). The later means that B can be inferred with _ or have an specified type argument. Is there any other details or documentation for this? Is there a way to specify A in the former case?