public interface RT
Request Types, again defined as strings for easier use with
mdf_message_add_numeric ().
IMAGE is for requesting a snapshot of the current values STREAM is for
requesting streaming data FULL is for requesting both IMAGE and STREAM
When requesting MDF_RC_INSREF, this field is used to tell the number of
instrument references wanted. If the number of requested references cannot be
fulfilled, for example due to account limits, the request will fail in full