Question about items function parameters

[happyduck-git]

Hello! I was reading Readme.md and got a question about how should I understand those parameters.
For this function,
rx.items(cellIdentifier:String)
I guess the parameter name is cellIdentifier and the value is String type.

rx.items(cellIdentifier:String:Cell.Type:_:)
But for this function, why there is ': ' right after 'String' and why there is '_:' at the end? I saw the source code and it seems like this functions parameters are actually rx.items(cellIdentifier:String cellType: Cell.Type) ?

Could I get a help for understanding about this by any chance? Thank you