A marker is to mark a point in time, or a period of time, inside a record.
Markers are always linked to a record, so you must start a record before you add a marker. You can't add nor update a marker after the record is stopped. You can associate some data to each marker, like a value and a label.
There are two types of markers:
An instance marker is to mark a point in time. You call injectMarker to create the marker at a specific timestamp.