Respiration rate messages store the breathing rate measured throughout the day and night.
timestampDateTimeMoment the message refers to, in UTC. FIT stores it as seconds since the FIT epoch (December 31, 1989, 00:00:00 UTC); decoders usually expose it as a regular date.
Respiration rate. Negative values flag invalid readings, large motion or off-wrist periods.
mesgNumStringGlobal message number identifying the kind of message. Decoders use it to know which profile to apply to the data.