Amazfit Watch faces

I have an Amazfit fit bit lite watch. the watch face I have chosen shows three numbers in the lower right quarter. The first, opposite a pair of shoes, shows the number of steps. The last, opposite the symbol of a heart, shows the heartbeat. But between the two is another symbol, looking like an upside-down heart with a black area in the middle, and beside it there are some whole numbers, at the moment, 15. But what does this symbol mean and what is it measuring?
Thanks for any info.

A lot of watchfaces use a fire or flame symbol to represent "burning" calories.
If you squint, does it look like a fire?

Yes, it could be that.
Do the calories only increase with "steps" or does it take into account biking or swimming or other exercise?