VMS Help TCPIP Services, Programming Interfaces, Sockets API, $QIO, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The Queue I/O Request service operates only on assigned I/O
    channels and only from access modes that are equal to or more
    privileged than the access mode from which the original channel
    assignment was made.
    For TCP/IP Services, $QIO uses the following system resources:
    o  The process's AST limit (ASTLM) quota, if an AST service
       routine is specified.
    o  System dynamic memory, which is required to queue the I/O
       request. System dynamic memory requirements are protocol
       specific.
    o  Additional memory, on a device-dependent basis.
    For $QIO, completion can be synchronized as follows:
    o  By specifying the astadr argument to have an AST routine
       execute when the I/O is completed.
    o  By calling the $SYNCH synchronize service to await completion
       of the I/O operation. (If you want your I/O operation to
       complete synchronously, use the $QIOW system service instead.)