Can be a component, function, design element, finishing detail etc. – but you must choose just one 
In the case of my one and only, the RDM, it is the Power Reserve indicator (no wonder it names the watch
):
I love to notice, from time to time, that the hand had moved automatically up and down and think it’s one of the most aesthetically pleasant among all PR indicators from all watch brands.
I look forward to knowing what yours is 
Marcelo