Those are annotations for method parameters and return values which were used by Distributed Objects. I say were because apparently there’s no sign of them in Apple’s documentation any longer. There used to be a Remote Messaging section in The Objective-C Programming Language document, which is still referenced by the Distributed Objects Programming Topics document.
- in: argument is an input argument only and won’t be referenced later
- out: argument is an output argument only, used to return a value by reference
- inout: argument is both an input and output argument
- const: the (pointer) argument is constant
- bycopy: instead of using a proxy/
NSDistantObject, pass or return a copy of the object - byref: use a proxy object (default)