in - passed from client to server

out - passed from server to client

optional - argument is optional

readonly - cannot be assigned to

partial_ignore - pointer to potentially uninitialized data

defaultvalue - default value for optional argument

retval - return value

ptr - full pointer, can be NULL and have aliases

ref - reference pointer, cannot be NULL and cannot have aliases

unique - unique pointer, can be NULL but cannot have aliases

force_allocate - always allocate dynamically

byte_count - size of referenced data

first_is - index of first array item

last_is - index of last array item

length_is - length of array

switch_is - discriminant of a union

pipe - stream between client and server

comm_status - failure code on communication error

fault_status - failure code on server error