A quick example on what $GPSTN messages are. Keep in mind that the sentence Identifier can also be displayed as $GNSTN. The message is the same but some GNSS receivers use this format.
Multiple Data ID.
This sentence is transmitted before each individual sentence where there is a need for the Listener to determine the exact source of data in the system. Examples might include dual-frequency depthsounding equipment or equipment that integrates data from a number of sources and produces a single output.
xx = Talker ID number, 00 to 99