Pressing the lower-center button will add a position marker on the map and in the log files.
If you wish to add the marker at your current position, first enter the
Map » Follow Me mode.
If you wish to add the marker at any other position, first navigate to it using
Map » Move Around mode and/or
Map » Zoom in/out mode.
Markers are shown on the map as little targets with various colours.
You can reload markers from a previous log file by following this procedure :
How to overlay a log or a track on the map.