Haven't checked this in detail, but I believe that Posix
applications (using the Posix API) cannot open MSG files,
but named pipes only. MPE apps using HPFOPEN should be
able to handle both types, though.

I also seem to recall that named pipes do not have the
"fifo buffering" of MSG files, i.e. you cannot write data
into a named pipe before there is a reader at the opposite
end. With MSG files you can stuff something in an pull it
out later (even from the same process).