Heart rate messages store heart rate data recorded by a chest strap while it was not connected to the watch (during swimming for instance), and merged back into the activity afterwards.
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.
Fractional part of the timestamp.
time256Float64Time with a 1/256 second resolution.
Heart rate values.
Moments of the heartbeats.
eventTimestamp12Byte[]Moments of the heartbeats, in a compact format decoded into eventTimestamp.
mesgNumStringGlobal message number identifying the kind of message. Decoders use it to know which profile to apply to the data.