Beat intervals messages store the time between consecutive heartbeats, measured by the optical sensor or a chest strap. They are the raw material for heart rate variability (HRV).
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.
Millisecond part of the timestamp.
Times between consecutive beats.
mesgNumStringGlobal message number identifying the kind of message. Decoders use it to know which profile to apply to the data.