After searching for some references to figure it out, -unfortunately- I could not find useful -and simple- description about understanding the differences between throws and rethrows. It is kind of confusing when try to understand how we should use them.

I would mention that I am kind of familiar with the -default- throws with its simplest form for propagating an error, as follows:

enum CustomError: Error {
    case potato
    case tomato
}

func throwCustomError(_ string: String) throws {
    if string.lowercased().trimmingCharacters(in: .whitespaces) == "potato" {
        throw CustomError.potato
    }

    if string.lowercased().trimmingCharacters(in: .whitespaces) == "tomato" {
        throw CustomError.tomato
    }
}

do {
    try throwCustomError("potato")
} catch let error as CustomError {
    switch error {
    case .potato:
        print("potatos catched") // potatos catched
    case .tomato:
        print("tomato catched")
    }
}

So far so good, but the problem arises when:

func throwCustomError(function:(String) throws -> ()) throws {
    try function("throws string")
}

func rethrowCustomError(function:(String) throws -> ()) rethrows {
    try function("rethrows string")
}

rethrowCustomError { string in
    print(string) // rethrows string
}

try throwCustomError { string in
    print(string) // throws string
}

what I know so far is when calling a function that throws it has to be handled by a try, unlike the rethrows. So what?! What is logic that we should follow when deciding to use throws or rethrows?