Difference between "mark" and "marker"

In a diagram, I added small visual elements to identify specific objects. Should I call these things (visual) marks or (visual) markers? What is the difference? Is there a better term?

According to dictionary.com a mark is a a visible impression or trace on something and a marker is something used as a mark or indication. However, this does not answer my question because the definition of marker is somehow self-referencing: is something used as a mark a mark or a marker?

Solution 1:

A mark is a visible impression or trace on something (as you found).

A marker is a mark with a specific meaning. For example, it can apply to a blob of paint indicating a part has been inspected: the blob is a mark, but its specific meaning turns it into a marker.

Your marks have a specific purpose to identify the various objects. Thus they are markers.