Differences between [in, out] and [out, retval] in COM IDL definitions

Question

In some of the IDL I work with I have noticed that there are 2 conventions for marking return values in methods - [in, out] and [out, retval].

Answer 1

Another difference is how this will show up in a PIA in .NET code.

[out, ret] will show up as a return value in .NET.

[out] will show up an argument on the method in .NET.